feat: add health-check in documentation and enrich response [MCP-517]

[gusanthiago]

Proposed Changes

Closes #1223.

The README never documented that the health check path is /health, so I added a
"Monitoring Server (Health Check & Metrics)" section covering:

• The health-check → /health and metrics → /metrics mappings.
• A runnable example, plus the comma-separated --monitoringServerFeatures health-check,metrics form.
• That the server only starts when both monitoringServerHost and monitoringServerPort are set.

I also enriched the /health response. It still returns status: "ok" (nothing breaks),
but now includes version, uptimeSeconds, and timestamp, and sets Cache-Control: no-store:

{ "status": "ok", "version": "1.13.0", "uptimeSeconds": 42, "timestamp": "2026-06-20T12:00:00.000Z" }

I didn't add a DB/readiness check — connections are per-session, so there's no single
"is the DB up" answer without a bigger change. Happy to drop the response changes if you'd
prefer this stay docs-only.

Testing

• Updated monitoringServer.test.ts for the new fields and header (9/9 passing).
• Verified against a local build:

  $ curl -D - http://127.0.0.1:8080/health
  HTTP/1.1 200 OK
  Cache-Control: no-store
  {"status":"ok","version":"1.13.0","uptimeSeconds":4,"timestamp":"2026-06-21T02:17:13.884Z"}
  

Checklist

• I have signed the MongoDB CLA
• New and existing unit tests pass locally with my changes
• I have added/updated documentation as needed