feat: ✨ add opentelemetry instrumentation

[Tochemey]

• Added OpenTelemetry tracing tests for HTTP transport and client when a tracer is configured, ensuring spans are emitted with correct statuses/attributes and server handler wiring is active. Introduced tracing-focused test stubs for transport.
• Implemented full GroupMetric registration and accessors, aligning metrics with GroupStats fields and adding a peer-latency gauge.
• Added a metrics plumbing suite in stats_test.go to cover meter provider defaults/overrides and to exercise success/error paths for group/cache instruments and NewGroup metric registration failures; added recording/failing meter fakes.
• Added NewGroup tests that validate metric registration succeeds (and instruments/counts are correct) and that failures propagate.

@thrawn01 whenever free kindly review this PR

[thrawn01]

Thank you for spending the time to implement this, this is great!