grafana: Add simple dashboard for Nginx cache and HTTP/TLS version stats

It has three panels showing "Cache hit ratio", "HTTP version" and "TLS
version".

The metrics are generated with Loki recording rules, in the two previous
commits.

roles/grafana/files/dashboards/nginx-stats.json

+{
+  "annotations": {
+    "list": [
+      {
+        "builtIn": 1,
+        "datasource": {
+          "type": "grafana",
+          "uid": "-- Grafana --"
+        },
+        "enable": true,
+        "hide": true,
+        "iconColor": "rgba(0, 211, 255, 1)",
+        "name": "Annotations & Alerts",
+        "type": "dashboard"
+      }
+    ]
+  },
+  "description": "https://gitlab.archlinux.org/archlinux/infrastructure/-/merge_requests/865",
+  "editable": true,
+  "fiscalYearStartMonth": 0,
+  "graphTooltip": 0,
+  "id": 32,
+  "links": [],
+  "panels": [
+    {
+      "datasource": {
+        "type": "prometheus",
+        "uid": "PBFA97CFB590B2093"
+      },
+      "description": "Only cacheable requests are included (e.g. {fastcgi,proxy}_cache is configured)",
+      "fieldConfig": {
+        "defaults": {
+          "color": {
+            "mode": "palette-classic",
+            "seriesBy": "last"
+          },
+          "custom": {
+            "axisBorderShow": false,
+            "axisCenteredZero": false,
+            "axisColorMode": "text",
+            "axisLabel": "",
+            "axisPlacement": "auto",
+            "barAlignment": 0,
+            "barWidthFactor": 0.6,
+            "drawStyle": "line",
+            "fillOpacity": 100,
+            "gradientMode": "opacity",
+            "hideFrom": {
+              "legend": false,
+              "tooltip": false,
+              "viz": false
+            },
+            "insertNulls": false,
+            "lineInterpolation": "linear",
+            "lineStyle": {
+              "fill": "solid"
+            },
+            "lineWidth": 1,
+            "pointSize": 5,
+            "scaleDistribution": {
+              "type": "linear"
+            },
+            "showPoints": "auto",
+            "spanNulls": false,
+            "stacking": {
+              "group": "A",
+              "mode": "none"
+            },
+            "thresholdsStyle": {
+              "mode": "off"
+            }
+          },
+          "mappings": [],
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "green",
+                "value": null
+              },
+              {
+                "color": "red",
+                "value": 80
+              }
+            ]
+          },
+          "unit": "percentunit"
+        },
+        "overrides": []
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 24,
+        "x": 0,
+        "y": 0
+      },
+      "id": 1,
+      "options": {
+        "legend": {
+          "calcs": [
+            "mean"
+          ],
+          "displayMode": "table",
+          "placement": "right",
+          "showLegend": true,
+          "sortBy": "Mean",
+          "sortDesc": true
+        },
+        "tooltip": {
+          "mode": "multi",
+          "sort": "desc"
+        }
+      },
+      "pluginVersion": "11.3.1",
+      "targets": [
+        {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "PBFA97CFB590B2093"
+          },
+          "editorMode": "code",
+          "exemplar": false,
+          "expr": "instance_cache_status:requests:rate1m{instance=\"$instance\",cache_status!=\"\"} / ignoring(cache_status) group_left sum by (instance) (instance_cache_status:requests:rate1m{instance=\"$instance\",cache_status!=\"\"})",
+          "instant": false,
+          "legendFormat": "{{cache_status}}",
+          "range": true,
+          "refId": "A"
+        }
+      ],
+      "title": "Cache hit ratio",
+      "type": "timeseries"
+    },
+    {
+      "datasource": {
+        "type": "prometheus",
+        "uid": "PBFA97CFB590B2093"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "color": {
+            "mode": "palette-classic",
+            "seriesBy": "last"
+          },
+          "custom": {
+            "axisBorderShow": false,
+            "axisCenteredZero": false,
+            "axisColorMode": "text",
+            "axisLabel": "",
+            "axisPlacement": "auto",
+            "barAlignment": 0,
+            "barWidthFactor": 0.6,
+            "drawStyle": "line",
+            "fillOpacity": 100,
+            "gradientMode": "opacity",
+            "hideFrom": {
+              "legend": false,
+              "tooltip": false,
+              "viz": false
+            },
+            "insertNulls": false,
+            "lineInterpolation": "linear",
+            "lineStyle": {
+              "fill": "solid"
+            },
+            "lineWidth": 1,
+            "pointSize": 5,
+            "scaleDistribution": {
+              "type": "linear"
+            },
+            "showPoints": "auto",
+            "spanNulls": false,
+            "stacking": {
+              "group": "A",
+              "mode": "none"
+            },
+            "thresholdsStyle": {
+              "mode": "off"
+            }
+          },
+          "mappings": [],
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "green",
+                "value": null
+              },
+              {
+                "color": "red",
+                "value": 80
+              }
+            ]
+          },
+          "unit": "percentunit"
+        },
+        "overrides": []
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 24,
+        "x": 0,
+        "y": 8
+      },
+      "id": 2,
+      "options": {
+        "legend": {
+          "calcs": [
+            "mean"
+          ],
+          "displayMode": "table",
+          "placement": "right",
+          "showLegend": true,
+          "sortBy": "Mean",
+          "sortDesc": true
+        },
+        "tooltip": {
+          "mode": "multi",
+          "sort": "desc"
+        }
+      },
+      "pluginVersion": "11.3.1",
+      "targets": [
+        {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "PBFA97CFB590B2093"
+          },
+          "editorMode": "code",
+          "exemplar": false,
+          "expr": "sum by (instance, http_version) (instance_http_version_tls_version_tls_cipher:requests:rate1m{instance=\"$instance\",http_version!=\"\"}) / ignoring(http_version) group_left sum by (instance) (instance_http_version_tls_version_tls_cipher:requests:rate1m{instance=\"$instance\",http_version!=\"\"})",
+          "instant": false,
+          "legendFormat": "{{http_version}}",
+          "range": true,
+          "refId": "A"
+        }
+      ],
+      "title": "HTTP version",
+      "type": "timeseries"
+    },
+    {
+      "datasource": {
+        "type": "prometheus",
+        "uid": "PBFA97CFB590B2093"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "color": {
+            "mode": "palette-classic",
+            "seriesBy": "last"
+          },
+          "custom": {
+            "axisBorderShow": false,
+            "axisCenteredZero": false,
+            "axisColorMode": "text",
+            "axisLabel": "",
+            "axisPlacement": "auto",
+            "barAlignment": 0,
+            "barWidthFactor": 0.6,
+            "drawStyle": "line",
+            "fillOpacity": 100,
+            "gradientMode": "opacity",
+            "hideFrom": {
+              "legend": false,
+              "tooltip": false,
+              "viz": false
+            },
+            "insertNulls": false,
+            "lineInterpolation": "linear",
+            "lineStyle": {
+              "fill": "solid"
+            },
+            "lineWidth": 1,
+            "pointSize": 5,
+            "scaleDistribution": {
+              "type": "linear"
+            },
+            "showPoints": "auto",
+            "spanNulls": false,
+            "stacking": {
+              "group": "A",
+              "mode": "none"
+            },
+            "thresholdsStyle": {
+              "mode": "off"
+            }
+          },
+          "mappings": [],
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "green",
+                "value": null
+              },
+              {
+                "color": "red",
+                "value": 80
+              }
+            ]
+          },
+          "unit": "percentunit"
+        },
+        "overrides": []
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 24,
+        "x": 0,
+        "y": 16
+      },
+      "id": 3,
+      "options": {
+        "legend": {
+          "calcs": [
+            "mean"
+          ],
+          "displayMode": "table",
+          "placement": "right",
+          "showLegend": true,
+          "sortBy": "Mean",
+          "sortDesc": true
+        },
+        "tooltip": {
+          "mode": "multi",
+          "sort": "desc"
+        }
+      },
+      "pluginVersion": "11.3.1",
+      "targets": [
+        {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "PBFA97CFB590B2093"
+          },
+          "editorMode": "code",
+          "exemplar": false,
+          "expr": "sum by (instance, tls_version) (instance_http_version_tls_version_tls_cipher:requests:rate1m{instance=\"$instance\",tls_version!=\"\"}) / ignoring(tls_version) group_left sum by (instance) (instance_http_version_tls_version_tls_cipher:requests:rate1m{instance=\"$instance\",tls_version!=\"\"})",
+          "instant": false,
+          "legendFormat": "{{tls_version}}",
+          "range": true,
+          "refId": "A"
+        }
+      ],
+      "title": "TLS version",
+      "type": "timeseries"
+    }
+  ],
+  "preload": false,
+  "schemaVersion": 40,
+  "tags": [],
+  "templating": {
+    "list": [
+      {
+        "current": {
+          "text": "wiki.archlinux.org",
+          "value": "wiki.archlinux.org"
+        },
+        "datasource": {
+          "type": "prometheus",
+          "uid": "PBFA97CFB590B2093"
+        },
+        "definition": "label_values(instance_cache_status:requests:rate1m,instance)",
+        "includeAll": false,
+        "label": "Instance",
+        "name": "instance",
+        "options": [],
+        "query": {
+          "qryType": 1,
+          "query": "label_values(instance_cache_status:requests:rate1m,instance)",
+          "refId": "PrometheusVariableQueryEditor-VariableQuery"
+        },
+        "refresh": 1,
+        "regex": "",
+        "type": "query"
+      }
+    ]
+  },
+  "time": {
+    "from": "now-6h",
+    "to": "now"
+  },
+  "timepicker": {},
+  "timezone": "browser",
+  "title": "Nginx stats",
+  "uid": "cdvt95bi4660wc",
+  "version": 16,
+  "weekStart": ""
+}
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