Diagnostics and metrics surface upgrades for admin workflows

[MPCoreDeveloper]

Summary

Upgrade diagnostics surfaces for admin and monitoring workflows.

Why

Developers and operators need reliable health and metrics insight without deep internals access.

Scope

• Expand metrics coverage (connections, queries, latency, errors, protocol-level counters).
• Improve health endpoints with richer status payloads.
• Document Prometheus scraping and dashboards baseline.

Implementation Plan

1. Define diagnostics KPI set for server/admin use.
2. Extend metrics collector and endpoint payloads.
3. Add tests for metrics/health contracts.
4. Publish observability setup guide.

Acceptance Criteria

• Metrics endpoints provide actionable server telemetry.
• Health endpoint supports quick triage.
• Docs include sample dashboard queries/panels.

Dependencies

• Independent but supports all UI/tooling tracks.