Support OpenTelemetry Traces and connection metrics in gRPC stubs

### 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](https://opentelemetry.io/docs/concepts/signals/traces/) support.
It would also be helpful if the generated service stubs and runtime support classes published connection-relevant [OpenTelemetry metric](https://opentelemetry.io/docs/concepts/signals/metrics/) values.

### Alternatives

_No response_