Add 100G volume to monitoring

It was getting very cramped with only a few MBs left. It appears currently we only get about 6 months worth of data out of 20G
so 100G seems reasonable.

tf-stage1/archlinux.tf

@@ -494,3 +494,9 @@ resource "hcloud_volume" "homedir" {
   size      = 100
   server_id = hcloud_server.machine["homedir.archlinux.org"].id
 }
+
+resource "hcloud_volume" "monitoring" {
+  name      = "monitoring"
+  size      = 100
+  server_id = hcloud_server.machine["monitoring.archlinux.org"].id
+}