Metadata type changed from `list` to `tuple` affecting modification via interceptor

[ohmayr]

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))

[daniel-sanche]

So it looks like there are two ways to address this:

• replace the tuple with a list in gapic_v1.method
• update that _add_metadata call to something like:

def _add_metadata(self, client_call_details):
        if client_call_details.metadata is not None:
            client_call_details.metadata = [(self._key, self._value), *client_call_details.metadata]

The type annotations for metadata seem to only guarantee a Sequence for metadata passed in, and doesn't explicitly say anything about what format the metadata will have when passed to the callable. IMO, I'd say that the bug is in the tests, which probably shouldn't be assuming a specific sequence type, and should assume metadata output will have the same structure as the input

But if we find other places are assuming a list and it's causing issues, we can always revert back to a list and change the API contract to be more specific here

[parthea]

I'm going to revert #527 as it's blocking our release for supporting protobuf 5.x . Please open a new PR to restore the feature with the suggested fix from #660 (comment)

[parthea]

Bumping to P2 as #527 will be reverted in main