nixos-module: add collectd postgresql metrics

nixos-module.nix

@@ -157,6 +157,30 @@ in
         GRANT ALL PRIVILEGES ON DATABASE caveman TO ${dbUser};
       '';
     };
+    services.collectd.plugins.postgresql = ''
+      <Query hosts_with_tokens>
+        Statement "select count(distinct host) from instance_tokens;"
+        <Result>
+          Type gauge
+          InstancePrefix "unique"
+          ValuesFrom "count"
+        </Result>
+      </Query>
+      <Query total_tokens>
+        Statement "select count(*) from instance_tokens;"
+        <Result>
+          Type gauge
+          InstancePrefix "total"
+          ValuesFrom "count"
+        </Result>
+      </Query>
+
+      <Database ${config.networking.hostName}>
+        Param database "caveman"
+        Query total_tokens
+        Query hosts_with_tokens
+      </Database>
+    '';
 
     systemd.services.caveman-hunter = lib.mkIf cfg.hunter.enable {
       wantedBy = [ "multi-user.target" ];