Require a token to scrape /metrics

The metrics endpoint fans out a gather to every connected host, so
anyone who could reach the controller could read cluster statistics and
put load on all hosts at will. Scraping now needs a user token holding
the new core.controller.metrics permission. The token is accepted via
Authorization: Bearer as well as X-Access-Token because Prometheus can
only send the former. Concurrent scrapes share one gather so hosts see
at most one metrics request at a time.

The three connection gauges in controller.ts now no-op before the
controller exists, like the other gauges there, so the endpoint can be
tested against the mock controller.

Closes #1009

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/managing-a-cluster.md

@@ -274,3 +274,28 @@ Remote updates are enabled by default but can be disabled on a per-machine basis
     controller> config set controller.allow_plugin_install true/false
 
 Remote installs are disabled by default but can be enabled on a per-machine basis at the owner's discretion. For security reasons, this configuration setting cannot be changed remotely. To modify it, run the above commands locally while the remote is offline.
+
+
+## Prometheus Metrics
+
+The controller exports metrics for the whole cluster on the `/metrics` endpoint.
+Fetching it requires a token for a user with the `core.controller.metrics` permission, sent either as an `Authorization: Bearer <token>` header or as an `X-Access-Token` header.
+Requests without a valid token get a 401 response and requests for users lacking the permission get a 403 response.
+
+Create a role with just this permission, a user to hold it, and generate a token for that user:
+
+    ctl> role create Prometheus --permissions core.controller.metrics
+    ctl> user create prometheus
+    ctl> user set-roles prometheus Prometheus
+    $ npx clusteriocontroller bootstrap generate-user-token prometheus
+
+Then pass the token in the scrape job for the controller in `prometheus.yml`:
+
+    scrape_configs:
+      - job_name: clusterio
+        authorization:
+          credentials: <token>
+        static_configs:
+          - targets: ["localhost:8080"]
+
+Concurrent scrapes share a single round of metrics gathering from the hosts, so multiple scrapers polling the controller do not multiply the load on the hosts.