#4252)

* fix: Refine connecitivity metrics to capture RPCs with no response headers

* test fix

Most transactions that exceed the mutation limit for an atomic transaction will fail with the error "The transaction contains too many mutations.". However, it is also possible that the transaction fails with the more generic error message "Transaction resource limits exceeded". This error did not trigger a retry of the statement using a PDML transaction.

Fixes #4253

…v3.54.2 (#4261)

- Send a RequestID to Spanner for each request
- Make sure that the attempt number of the RequestID is increased
  if the RPC is retried.
- Include the RequestID in every error that is thrown due to an error
  that is returned by Spanner.

This PR enables the gRPC-GCP channel pool extension by default for Cloud Spanner Java client.

**What's Changing for Customers**

**Before this change**

- gRPC-GCP extension was disabled by default
- Default number of channels: 4
- Channel pooling was handled by GAX

**After this change**

- gRPC-GCP extension is enabled by default
- Default number of channels: 8
- Channel pooling is handled by gRPC-GCP extension

**Benefits of gRPC-GCP**

- **Improved resilience:** When a network connection fails on a particular channel, operations can be automatically retried on a different gRPC channel
- **Better channel management:** gRPC-GCP provides more sophisticated channel affinity and load balancing

**How to Disable gRPC-GCP (Switch Back to GAX Channel Pool)**

If you need to disable gRPC-GCP and use the previous GAX channel pooling behavior, use the `disableGrpcGcpExtension()` method:
```
SpannerOptions options = SpannerOptions.newBuilder()
    .setProjectId("my-project")
    .disableGrpcGcpExtension()
    .build();
```

When disabled, the default number of channels reverts to 4 (the previous default).

**When You Might Want to Disable gRPC-GCP**

- **Maintaining previous behavior:** If you want to keep the exact same behavior as before this change (GAX channel pool with 4 default channels).
- **Troubleshooting**: If you experience any unexpected behavior, disabling gRPC-GCP can help isolate whether the issue is related to the channel pooling mechanism.

🤖 I have created a release *beep* *boop*
---


### Updating meta-information for bleeding-edge SNAPSHOT release.

---
This PR was generated with [Release Please](https://togithub.com/googleapis/release-please). See [documentation](https://togithub.com/googleapis/release-please#release-please).

* feat: add support of dynamic channel pooling

* set initial pool size to 0 make dynamic channel pool work

* chore: generate libraries at Tue Dec 16 06:00:50 UTC 2025

* bump grpc-gcp-java version

* fix test

* incorporate suggestions

* chore: generate libraries at Tue Dec 16 09:56:45 UTC 2025

* make dynamic channel pool default disabled

* add verifySameChannelId back and fix partitionDML test

* support setting GcpChannelPoolOptions directly

---------

Co-authored-by: cloud-java-bot <[email protected]>

* chore(main): release 6.105.0

* chore: generate libraries at Tue Dec 16 17:46:24 UTC 2025

---------

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: cloud-java-bot <[email protected]>