fix(spanner): record streaming operation completion when stream finishes

[sinhasubham]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request refactors the Spanner client's metrics tracking to ensure that streaming operation latencies are recorded immediately upon stream completion and exclude consumer processing delays. This is achieved by tracking stream completion in both synchronous and asynchronous snapshot iterators and recording metrics accordingly. However, the reviewer identified a critical issue in metrics_interceptor.py where the code attempts to check for a last attribute on the yielded stream items. Since the production PartialResultSet protobuf messages do not contain a last field, this check will always evaluate to False and act as dead code. It is recommended to remove these redundant checks.

[gemini-code-assist]

In production, the item yielded by the gRPC response iterator is a PartialResultSet protobuf message (or a proto-plus wrapper). The PartialResultSet proto definition does not contain a last field. Therefore, getattr(item, "last", False) will always return False in production, making this check dead code that never executes.

The integration tests pass only because they fully consume the stream (e.g., using list(results) or for _ in results), which raises StopIteration and triggers _record_metrics() in the except StopIteration block.

Since there is no last field in the Spanner PartialResultSet proto, we should remove this check to avoid dead code and false assumptions about the protobuf structure.

        return item

[gemini-code-assist]

Similarly to the synchronous version, the item yielded by the async iterator is a PartialResultSet protobuf message, which does not have a last field. Thus, getattr(item, "last", False) will always return False in production. We should remove this check.

        return item