Show the date when the certificate expires

The value of the expr is not really useful as of now, but if we show the
value of probe_ssl_earliest_cert_expiry it should show the date when the
cert expires.

roles/prometheus/files/node.rules.yml

@@ -420,13 +420,13 @@ groups:
           summary: "Blackbox probe HTTP failure (instance {{ $labels.instance }})"
           description: "HTTP status code is not 200-399\n  VALUE = {{ $value }}\n  LABELS: {{ $labels }}"
       - alert: BlackboxSslCertificateWillExpireSoon
-        expr: probe_ssl_earliest_cert_expiry - time() < 86400 * 25
+        expr:  probe_ssl_earliest_cert_expiry and probe_ssl_earliest_cert_expiry - time() < 86400 * 25
         for: 5m
         labels:
           severity: critical
         annotations:
           summary: "Blackbox SSL certificate will expire soon (instance {{ $labels.instance }})"
-          description: "SSL certificate expires in 25 days\n  VALUE = {{ $value }}\n  LABELS: {{ $labels }}"
+          description: "SSL certificate expires in 25 days\n  VALUE = {{ $value | humanizeTimestamp}}\n  LABELS: {{ $labels }}"
 
   - name: rebuilderd
     interval: 15m