[ruby] Seeking review: fix for interceptor metadata dropped on streaming calls (#42073)

[Daniel Christo]

Hi all,

I've opened a PR to fix a long-standing Ruby bug where client interceptor metadata is silently dropped on streaming calls when return_op: true is used. This affects client_streamer, server_streamer, and bidi_streamer — meaning interceptors that add auth headers (or any metadata) don't work correctly for streaming RPCs.

This was originally reported 6 years ago in #22448, and a fix was authored by @apolcyn in #25072, but both were closed by the stale bot before being merged. My PR revives that same approach and includes tests.

PR: https://github.com/grpc/grpc/pull/42073
Original issue: https://github.com/grpc/grpc/issues/22448

Would really appreciate a review from anyone familiar with the Ruby client. Happy to address any feedback.

Thanks!