add description of prometheus service

clanServices/prometheus/README.md

@@ -0,0 +1,38 @@
+This module enables collecting metrics from machines in clan, using Prometheus.
+
+There are two roles:
+
+- A `target` role for machines on which to collect and export metrics.
+- A `scraper` roles for machines that fetch metrics from `target` machines and
+  store them in the long term.
+
+
+```nix
+inventory = {
+
+  machines = {
+    server01.tags.server = {};
+    server02.tags.server = {};
+    metrics.tags.server = {}; # metrics collector
+  };
+
+  instances = {
+    prometheus = {
+      module.name = "@rpqt/prometheus";
+      module.input = "self";
+
+      roles.scraper.machines."metrics" = {};
+
+      # Collect metrics on all servers
+      roles.target.tags.server = {
+        settings = {
+          exporters = {
+            # Enable the node-exporter metrics source
+            node.enabledCollectors = [ "systemd" ];
+          };
+        };
+      };
+    };
+  };
+};
+```