feat(observability): Add comprehensive monitoring with Prometheus and OpenTelemetry

- Add Prometheus metrics for HTTP, MCP tools, Nextcloud API, OAuth, vector sync, and DB operations
- Add OpenTelemetry distributed tracing with OTLP export
- Add structured JSON logging with trace context correlation
- Add ObservabilityMiddleware for automatic HTTP instrumentation
- Add app_name attribute to all client classes for per-app metrics
- Add configuration for metrics, tracing, and logging via environment variables
- Add documentation in docs/observability.md
- Fix graceful degradation when tracing is disabled (default state)
- Fix uvicorn logging configuration to use observability formatters

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/client/users.py

@@ -7,6 +7,8 @@ from nextcloud_mcp_server.models.users import UserDetails
 class UsersClient(BaseNextcloudClient):
     """Client for Nextcloud User API operations."""
 
+    app_name = "users"
+
     def _get_user_headers(
         self, additional_headers: Optional[Dict[str, str]] = None
     ) -> Dict[str, str]: