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
* first round cleanup based on go-critic
* more issues resolved for ifelsechain and unlambda checks
* updated from master and gocritic found a new ifElseChain issue
* disable appendAssign. i reports false positives
* re-enabled go-critic appendAssign and disabled lint check at code line level
* fixed go-critic lint error
* fixed // nolint add gocritic specifically
