What: we move api keys out of the grpc connection-level metadata on the client side and into the request protobufs directly. the server side still supports both mechanisms for backwards compatibility.
Why: dRPC won't support connection-level metadata. the only thing we currently use connection-level metadata for is api keys. we need to move all information needed by a request into the request protobuf itself for drpc support. check out the .proto changes for the main details.
One fun side-fact: Did you know that protobuf fields 1-15 are special and only use one byte for both the field number and type? Additionally did you know we don't use field 15 anywhere yet? So the new request header will use field 15, and should use field 15 on all protobufs going forward.
Please describe the tests: all existing tests should pass
Please describe the performance impact: none
