Add Prometheus metrics endpoint and Grafana dashboard for brightstaff (#904)

docs/source/guides/observability/monitoring.rst

@@ -75,3 +75,54 @@ are some sample configuration files for both, respectively.
         isDefault: true
         access: proxy
         editable: true
+
+Brightstaff metrics
+~~~~~~~~~~~~~~~~~~~
+
+In addition to Envoy's stats on ``:9901``, the brightstaff dataplane
+process exposes its own Prometheus endpoint on ``0.0.0.0:9092`` (override
+with ``METRICS_BIND_ADDRESS``). It publishes:
+
+* HTTP RED — ``brightstaff_http_requests_total``,
+  ``brightstaff_http_request_duration_seconds``,
+  ``brightstaff_http_in_flight_requests`` (labels: ``handler``, ``method``,
+  ``status_class``).
+* LLM upstream — ``brightstaff_llm_upstream_requests_total``,
+  ``brightstaff_llm_upstream_duration_seconds``,
+  ``brightstaff_llm_time_to_first_token_seconds``,
+  ``brightstaff_llm_tokens_total`` (labels: ``provider``, ``model``,
+  ``error_class``, ``kind``).
+* Routing — ``brightstaff_router_decisions_total``,
+  ``brightstaff_router_decision_duration_seconds``,
+  ``brightstaff_routing_service_requests_total``,
+  ``brightstaff_session_cache_events_total``.
+* Process & build — ``process_resident_memory_bytes``,
+  ``process_cpu_seconds_total``, ``brightstaff_build_info``.
+
+A self-contained Prometheus + Grafana stack is shipped under
+``config/grafana/``. With Plano already running on the host, bring it up
+with one command:
+
+.. code-block:: bash
+
+    cd config/grafana
+    docker compose up -d
+    open http://localhost:3000   # admin / admin (anonymous viewer also enabled)
+
+Grafana auto-loads the Prometheus datasource and the brightstaff
+dashboard (look under the *Plano* folder). Prometheus scrapes the host's
+``:9092`` and ``:9901`` via ``host.docker.internal``.
+
+Files:
+
+* ``config/grafana/docker-compose.yaml`` — one-command Prom + Grafana
+  stack with provisioning.
+* ``config/grafana/prometheus_scrape.yaml`` — complete Prometheus config
+  with ``envoy`` and ``brightstaff`` scrape jobs (mounted by the
+  compose).
+* ``config/grafana/brightstaff_dashboard.json`` — 19-panel dashboard
+  across HTTP RED, LLM upstream, Routing service, and Process & Envoy
+  link rows. Auto-provisioned by the compose; can also be imported by
+  hand via *Dashboards → New → Import*.
+* ``config/grafana/provisioning/`` — Grafana provisioning files for the
+  datasource and dashboard provider.