Underscores are commonly used in names in generated code to avoid conflicts (this applies to all sorts of codegen, not just protobuf). You can easily have both Foo and FooBuilder messages in your protobuf. See also generated enum consts since day one of protobuf-gen-go.
Fair enough but now this name leaks out into code I have to type. I don't dig highly opinionated languages with tooling that complains at me over style guidelines breaking their own style guidelines to solve a problem that they themselves have created.
They painted themselves into a corner and their solution is for me to hit myself in the head with a hammer.
And in this case, perhaps, the resistance and the ugliness of the solution should have made the Protobuf authors to pause for a bit and rethink it.
Several years ago, I used gogoprotobuf instead of regular Go protobuf. It uses normal by-value structures instead of pointers everywhere, with generated code for s11n instead of reflection. It worked about 10x faster that the regular protobuf.