The following change d96eb5c introduces a breaking change by changing the type of metadata from a list to a tuple resulting in users to not be able to modify the metadata via an interceptor.

The following showcase tests in the generator break, since the interceptor can no longer modify the metadata (or assign it a new value) via this function:

def _add_metadata(self, client_call_details):
        if client_call_details.metadata is not None:
            client_call_details.metadata.append((self._key, self._value))