[opentelemetry-instrumentation-grpc] Support for metrics

[Sovietaced]

trafficstars

What problem do you want to solve?

Currently there are traces but no metrics. Having OTEL metrics for gRPC servers and clients is an incredibly useful feature.

Describe the solution you'd like

Metrics should be generated based on the OTEL specs similar to the go libraries that support it.

Additional Context

https://opentelemetry.io/docs/specs/semconv/rpc/rpc-metrics/

Would you like to implement a fix?

Yes