proposed fix: Merging operations causes duplicate key/value pairs in "map" fields

[sloretz]

Does this look like a change you'd accept?

I'd like to override the content-type annotation of a streaming API to text/event-stream for use with server-sent events. However, when I add an openapi.v3.operation annotation and generate an OpenAPI spec I get a schema that lists two response types for "200".

I'm assuming the problem is the proto file lists fields as repeated with Named... message types rather than map, and this proto.Merge appends those messages as if they were a list no matter what.

gnostic/cmd/protoc-gen-openapi/generator/generator.go

Lines 756 to 760 in e0e09f7

	// Merge any `Operation` annotations with the current
	extOperation := proto.GetExtension(method.Desc.Options(), v3.E_Operation)
	if extOperation != nil {
	proto.Merge(op, extOperation.(*v3.Operation))
	}

I think the fix would be to write a custom merge function that resolves conflicts. My preference would be to keep the last annotation. For now I've only updated the tests to show the behavior I want to change. How does it look to you?