Stabilize RPC semantic conventions

[trask]

RPC semantic conventions has gone Release Candidate!

We need 3 prototypes before we can mark it stable.

Current work underway:

• Java [@zeitlinger, @trask, @steverao]
  • gRPC
  • Apache Dubbo
  • Tracking issue: add RPC semconv stable opt-in opentelemetry-java-instrumentation#15871

[lmolkova]

Python prototype: open-telemetry/opentelemetry-python-contrib#4279

Call-outs:

• it does not seem to be possible to populate network.peer.address | port on client (they are recommended)
• it's possible to detect calls to missing methods/services without introducing side-effects, but it might need more hacking around gRPC python
• no metrics (not reported by current instrumentation)
• there were no span events for messages (and no issues asking for them) - interesting signal in favor of deprecating them