Add OpenTelemetry metrics instrumentation

[Zerpet]

This PR implements OTEL metrics for RabbitMQ Stream operations:

Metrics tracked:

• rabbitmq.stream.connections (up/down counter)
• rabbitmq.stream.published (counter)
• rabbitmq.stream.confirmed (counter)
• rabbitmq.stream.errored (counter)
• rabbitmq.stream.chunks (counter)
• rabbitmq.stream.chunk_size (histogram)
• rabbitmq.stream.consumed (counter)
• rabbitmq.stream.outstanding_publish_confirmations (up/down counter)

Implementation:

• Add MeterProvider configuration to EnvironmentOptions
• Instrument connection lifecycle, producer publish/confirm/error events, and consumer delivery events
• Include OTEL semantic conventions for attributes (messaging.system, server.address, server.port, messaging.destination.name, etc.)
• Default to noop provider when not configured

Testing:

• Add comprehensive unit tests with mock meter provider (21 tests)
• Add integration tests covering producer, consumer, connection, and error scenarios
• All tests avoid OTEL SDK dependency by using custom mocks

Example:

• Add examples/metrics demonstrating usage with stdout exporter

Running the example

To run the example, I recommend creating a Go workspace using go work as follows:

go work init
go work add .
go work add examples/metrics

This prevents the Go module from downloading the client from github, and instead use the local client code.

cd examples/metrics
go run metrics.go
# Remember to have a running rabbit

Closes: #26

[codecov]

Codecov Report

❌ Patch coverage is 86.98630% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.45%. Comparing base (4d1e770) to head (4fc83ed).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/stream/metrics.go	80.24%	8 Missing and 8 partials ⚠️
pkg/stream/environment.go	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
+ Coverage   83.36%   83.45%   +0.09%     
==========================================
  Files          28       29       +1     
  Lines        4238     4376     +138     
==========================================
+ Hits         3533     3652     +119     
- Misses        480      490      +10     
- Partials      225      234       +9     

Flag	Coverage Δ
unittests	83.45% <86.98%> (+0.09%)	⬆️

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[Gsantomaggio]

Please add some documentation to the README.md then we can merge! Thank you!