feat: track WebSocket requests in metrics

Add request tracking for EVENT and REQ messages to match gRPC behavior.
Dashboard now shows total/success/error counts for all requests.

- Add RecordRequest to MetricsRecorder interface
- Track timing and status in handleEvent and handleReq
- Record metrics with status: ok, error, unauthenticated, rate_limited
- Measure request duration for performance monitoring

WebSocket is the primary interface, so tracking these requests is critical
for understanding relay usage and performance.

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