docs: Add Metrics

Signed-off-by: Marcel Lauhoff <[email protected]>

docs/metrics.md

@@ -0,0 +1,57 @@
+[//]: <> (cSpell:ignoreRegExp /rgw[-_]\w+/)
+
+# Metrics
+
+S3GW metrics leverage the Ceph [Perf
+Counters](https://docs.ceph.com/en/latest/dev/perf_counters/) system
+already built into RGW.
+
+On top of Perf Counters we export additional 1D Prometheus-style
+histograms (Ceph's histograms are 2D) using a built in Prometheus
+endpoint part of the status frontend.
+
+The prometheus endpoint implementation is separate from [Ceph
+Exporter](https://github.com/digitalocean/ceph_exporter) or the Ceph
+Mgr [Prometheus
+Module](https://docs.ceph.com/en/latest/mgr/prometheus/).
+
+## Status Frontend
+
+s3gw supports an additional RGW frontend *status* offering a read-only
+view into the application's state. The frontend is optional and can be
+activated aside the beast frontend doing the S3 HTTP.
+
+Most status frontend pages are text-based and meant for human
+consumption. An exception is the Prometheus endpoint exporting perf
+counters as [Prometheus Exposition
+Format](https://prometheus.io/docs/instrumenting/exposition_formats/)
+
+To enable the status frontend including the Prometheus endpoint, add
+the `status` frontend to
+[`rgw_frontends`](https://docs.ceph.com/en/quincy/radosgw/config-ref/#confval-rgw_frontends)
+. On the command line this may look like:
+
+```sh
+--rgw-frontends 'beast port=7480, status bind=127.0.0.1 port=9090'
+```
+
+## Dashboard
+
+A S3GW dashboard is available on Grafana Hub with ID 19544.
+
+- [Grafana Dashboard](https://grafana.com/grafana/dashboards/19544-s3gw/)
+
+## Prometheus Scraping Setup
+
+The Prometheus endpoint is available on the status frontend address at `/prometheus`
+
+Example configuration:
+
+```yaml
+scrape_configs:
+  - job_name: 's3gw-status'
+    metrics_path: '/prometheus'
+    static_configs:
+      - targets:
+        - 127.0.0.1:9090
+```

mkdocs.yml

@@ -29,6 +29,7 @@ nav:
       - Testing the s3gw: "testing.md"
       - UI: "ui.md"
       - S3 API compatibility table: "s3-compatibility-table.md"
+      - Metrics: "metrics.md"
   - "Team Decisions":
       - Use markdown any decision records: "decisions/0000-use-markdown-any-decision-records.md"
       - Reorganize documentation: "decisions/0001-reorganize-documentation.md"