Add OpenTelemetry metrics support with semantic conventions

Implement metrics collection using the OpenTelemetry API following
OTEL semantic conventions for RabbitMQ messaging. The implementation
uses a pluggable MetricsCollector interface that allows users to
integrate with any metrics backend.

Key changes:
- Add MetricsCollector interface with NoOp and OTEL implementations
- Add OTELMetricsCollector using OTEL API (SDK kept separate)
- Implement OTEL semantic convention attributes (messaging.system,
  messaging.destination.name, messaging.operation.name, etc.)
- Refactor Environment constructors to use functional options pattern
- Add comprehensive unit and E2E tests in separate tests/otelmetrics module
- Add standalone example demonstrating OTEL metrics with stdout exporter
- Update METRICS_IMPLEMENTATION_GUIDE.md with OTEL semantic conventions

The metrics collected include:
- Connection/publisher/consumer gauges
- Published/consumed message counters
- Disposition counters (accepted/rejected/released/discarded/requeued)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Zerpet

.gitignore

@@ -30,6 +30,7 @@ go.work.sum
 # Left over files from tests
 coverage.txt
 .ci/ubuntu/log/rabbitmq.log
+docs/examples/otel_metrics/otel_metrics
 
 # Visual Studio Code
 .vscode/*