[WIP]: Add OpenTelemetry metrics support for XDS Client

[aadityaraj7769]

Summary

This PR adds OpenTelemetry (OTel) metrics instrumentation to the XDS Client.

Changes

• New Interface: Added XdsClientOtelMetricsProvider interface for collecting XDS client metrics via OpenTelemetry

  • Connection lifecycle events (connection lost, closed, reconnection)
  • Request/response tracking
  • Error tracking (resource not found, invalid resource)
  • Server latency measurements
  • Connection state monitoring
  • Initial wait time tracking for resource fetching

• No-op Implementation: Added NoOpXdsClientOtelMetricsProvider as default implementation when metrics are disabled

• Integration:

  • Integrated metrics provider into XdsClientImpl constructor with dependency injection pattern
  • Added metrics recording throughout XDS client lifecycle (connection events, resource updates, latency tracking)
  • Metrics provider passed to XdsClientJmx for JMX integration
  • Added builder support in D2ClientBuilder via setXdsClientOtelMetricsProvider()

• Metrics Tracked:

  • Counters: connection events, requests/responses, errors (not found/invalid resources), IRV sent count
  • Histogram: server latency
  • Gauges: connection state, active initial wait time

Testing

• Updated test fixtures to mock the new metrics provider

Backward Compatibility

• Fully backward compatible - defaults to no-op provider when not configured
• Uses constructor-based dependency injection following existing patterns (similar to XdsServerMetricsProvider)