Update docs & env examples for observability flags #2
Description
Goal\n----\nDocument new telemetry capabilities and configuration.\n\nAcceptance criteria\n-------------------\n1. README gains an Observability section covering Prometheus, OpenTelemetry & Loki.\n2. env.example updated with OBSERVABILITY_ENABLED, OTEL_* and LOKI_* variables.\n3. Provide sample docker-compose snippet showing endpoints when using mcp-observability stack.\n4. README badge or note on /metrics endpoint.\n