PR created by the Librarian CLI to initialize a release. Merging this PR will auto trigger a release.

Librarian Version: v1.0.0
Language Image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:8e2c32496077054105bd06c54a59d6a6694287bc053588e24debe6da6920ad91
<details><summary>google-cloud-spanner: 3.60.0</summary>

## [3.60.0](https://togithub.com/googleapis/python-spanner/compare/v3.59.0...v3.60.0) (2025-12-10)

### Features

* make built-in metrics enabled by default (#1459) ([64aebe7](https://togithub.com/googleapis/python-spanner/commit/64aebe7e))

* Exposing AutoscalingConfig in InstancePartition ([8b6f154](https://togithub.com/googleapis/python-spanner/commit/8b6f1540))

* add support for experimental host (#1452) ([9535e5e](https://togithub.com/googleapis/python-spanner/commit/9535e5e0))

* enable OpenTelemetry metrics and tracing by default (#1410) ([bb5095d](https://togithub.com/googleapis/python-spanner/commit/bb5095df))

* add cloud.region, request_tag and transaction_tag in span attributes (#1449) ([d37fb80](https://togithub.com/googleapis/python-spanner/commit/d37fb80a))

* Add QueryAdvisorResult for query plan (PiperOrigin-RevId: 832425466) ([e08260f](https://togithub.com/googleapis/python-spanner/commit/e08260fe))

* Add Send and Ack mutations for Queues (PiperOrigin-RevId: 832425466) ([e08260f](https://togithub.com/googleapis/python-spanner/commit/e08260fe))

* Add Spanner location API (#1457) (PiperOrigin-RevId: 833474957) ([e08260f](https://togithub.com/googleapis/python-spanner/commit/e08260fe))

### Bug Fixes

* Deprecate credentials_file argument ([8b6f154](https://togithub.com/googleapis/python-spanner/commit/8b6f1540))

* configure keepAlive time for gRPC TCP connections (#1448) ([efb2833](https://togithub.com/googleapis/python-spanner/commit/efb2833e))

* Provide Spanner Option to disable metrics (#1460) ([f1ebc43](https://togithub.com/googleapis/python-spanner/commit/f1ebc43b))

### Documentation

* Update description for the BatchCreateSessionsRequest and Session (PiperOrigin-RevId: 832425466) ([e08260f](https://togithub.com/googleapis/python-spanner/commit/e08260fe))

* Update description for the IsolationLevel (PiperOrigin-RevId: 832425466) ([e08260f](https://togithub.com/googleapis/python-spanner/commit/e08260fe))

</details>

Previously #1452
introduced changes to support python spanner client against spanner
experimental host endpoints over insecure communication

This PR extends those changes to support python spanner client
connections to experimental host endpoints over TLS / mTLS connections
as well. It also includes changes to run Integration Tests against
experimental hosts across all 3 modes of network communication
(plain-text, TLS, mTLS)

To run IT tests against experimental host set below variables
```
export SPANNER_EXPERIMENTAL_HOST=localhost:15000
```
For tls/mTLS set below additonal variables:
- (mTLS/TLS)
```
export CA_CERTIFICATE=/tmp/experimental_host/ca-certificates/ca.crt
```
- (mTLS)
```
export CLIENT_CERTIFICATE=/tmp/experimental_host/certs/client.crt
export CLIENT_KEY=/tmp/experimental_host/certs/client.key
```

Then we can run below command to tigger the tests:
```
python -m pytest -v -s --disable-warnings  tests/system/
```

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>

Previously googleapis/python-spanner#1452
introduced changes to support python spanner client against spanner
experimental host endpoints over insecure communication

This PR extends those changes to support python spanner client
connections to experimental host endpoints over TLS / mTLS connections
as well. It also includes changes to run Integration Tests against
experimental hosts across all 3 modes of network communication
(plain-text, TLS, mTLS)

To run IT tests against experimental host set below variables
```
export SPANNER_EXPERIMENTAL_HOST=localhost:15000
```
For tls/mTLS set below additonal variables:
- (mTLS/TLS)
```
export CA_CERTIFICATE=/tmp/experimental_host/ca-certificates/ca.crt
```
- (mTLS)
```
export CLIENT_CERTIFICATE=/tmp/experimental_host/certs/client.crt
export CLIENT_KEY=/tmp/experimental_host/certs/client.key
```

Then we can run below command to tigger the tests:
```
python -m pytest -v -s --disable-warnings  tests/system/
```

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>