BidiRpc missing timeout and retry arguments

[tswast]

Is your feature request related to a problem? Please describe.

Most (all?) generated methods contain the following arguments:

retry: google.api_core.retry.Retry = google.api_core.gapic_v1.method.DEFAULT,
timeout: float = None,
metadata: Sequence[Tuple[str, str]] = (),

I'd like to be able to pass these through from the method I'm wrapping. Currently, this is possible with metadata. See:

https://github.com/googleapis/python-api-core/blob/40f52bf1100cf56b7f9af267d210b8a72fc34f08/google/api_core/bidi.py#L279

But it does not appear to be possible with other arguments such as timeout and retry.

Describe the solution you'd like

Add retry and timeout as optional arguments to BidiRpc.

Perhaps if retry and timeout aren't available in older generated clients, some additional logic will be required to prevent breaking them? I recall they've been around for quite some time, though.

Describe alternatives you've considered

Perhaps an arbitrary **kwargs dictionary for extra arguments to pass to the _start_rpc method?

Additional context

• feat: add BigQueryWriteClient where append_rows returns a helper for writing rows python-bigquery-storage#278
• reat: Bi-Directional Streaming APIs could use BidiRpc base class by default #16450 -- I suspect this feature will need to be implemented before the generator can consider using BidiRpc by default.

[tswast]

I'm thinking maybe the **kwargs approach would be best, as that can more easily be made compatible with clients that don't have retry or timeout. It's also flexible in that it can include additional arguments in the future.

[parthea]

Transferring to google-cloud-python as the code for python-api-core will be migrated there in 1-3 weeks.