RPC message namespace is too generic

[lmolkova]

Names of RPC message attributes (message.id, message.compressed_size, etc) are too generic, while their semantics are very narrow.

https://github.com/open-telemetry/opentelemetry-specification/blob/b5c6a6dc48752a3ea740f08bed0c2975bae19673/semantic_conventions/trace/rpc.yaml#L193

We also have messaging.message.* namespace which contains messaging-specific attributes with quite similar names, but different semantics.

Assuming we create a registry of attributes and suggest reusing them across signals, we should either have a general-purpose message namespace, or, preferably, put RPC messages under rpc.message.*.

[Oberon00]

I think this is a legacy holdover from OpenCensus grpc streaming support