P0: Prometheus metrics hooks — no-op default + optional exporter #22
Description
Context: Implement minimal metrics hooks across DB and tool handlers with a no-op default recorder and optional Prometheus exporter controlled by env.
Acceptance criteria:
- Minimal Recorder interface with counters/histograms for DB ops and tools
- No-op default implementation active when METRICS_PROMETHEUS is unset
- Prometheus recorder enabled when METRICS_PROMETHEUS=true, served on METRICS_ADDR (default :9090)
- /metrics and /healthz endpoints exposed
- Latency histograms and totals for tool handlers and DB ops
- Lightweight result-size observations (sampled) and pool/stmt cache metrics
- Idempotent initialization (safe to call from multiple places)
- README updated with usage and recommended buckets