Update docs & env examples for observability flags

[pmcfadin]

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

[pmcfadin]

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