P2: Documentation: operator guide + dashboards — ops guide and Grafana

[ZanzyTHEbar]

Context: Metrics available with Prometheus exporter and healthz. Provide operator guide and sample dashboards.\n\nAcceptance criteria:\n- Operator guide covering envs, pooling, metrics knobs.\n- Example Prometheus/Grafana dashboards for db/tool latencies and pool gauges.\n- README/docs updated.