Implement open-telemetry instrumentation support.

Implement open-telemetry instrumentation support, modelling it after the corresponding [gRPC instrumentation package](https://github.com/open-telemetry/opentelemetry-go-contrib/tree/main/instrumentation/google.golang.org/grpc/otelgrpc).

- [ ] implement basic instrumentation support (interceptors for unary server and unary client): PR #145
- [ ] implement instrumentation for streaming

Start by reading the corresponding gRPC instrumentation package linked in the issue and review PR #145 for the proposed basic support. The work should cover unary server and client interceptors, followed by streaming instrumentation; completion means both checklist items are implemented and verified against the project's existing patterns.

Written by the indexing model from the issue text.