Support OpenTelemetry Traces and connection metrics in gRPC stubs

[jsync-swirlds]

Problem

The generated stubs and runtime support classes of the PBJ Helidon/gRPC plugin do not currently support trace context propagation or publish useful metrics. This limits observability for projects and systems using this feature.

Solution

The PBJ gRPC plugin and generated code interact with underlying transport much more directly than "business logic" code, and have clear access to useful metric and trace information.
It would be very helpful if the generated service stubs and runtime support classes were to manage context propagation for OpenTelemetry trace support.
It would also be helpful if the generated service stubs and runtime support classes published connection-relevant OpenTelemetry metric values.

Alternatives

No response

[akugal]

Maybe this could be part of a bigger Epic - to have a solution to extend PBJ generation with different aspects around business logic like: logging or metrics, network error handling, HTTP backpressure, circuit breaker, etc