Consume ni.measurementlink.discovery.v1.proto package and remove _internal.stubs for discovery#1216

dixonjoel merged 9 commits intomainni/measurement-plugin-python:mainfrom

users/jdixon/consume-ni-measurementlink-discovery-v1-protoni/measurement-plugin-python:users/jdixon/consume-ni-measurementlink-discovery-v1-protoCopy head branch name to clipboard