Upgrading timestamp_microseconds_value to timestamp_value.

Make built-in metrics enabled by default

This change inverts the logic for enabling built-in OpenTelemetry
metrics. Previously, metrics were disabled by default and could be
enabled by setting `ENABLE_SPANNER_METRICS_ENV_VAR=true`.

With this update, metrics are now enabled by default to provide better
out-of-the-box observability for users.

To disable metrics, users must now set the new environment variable:
`SPANNER_DISABLE_BUILTIN_METRICS=true`

The old `ENABLE_SPANNER_METRICS_ENV_VAR` is no longer used. Unit tests
have been updated to reflect this new opt-out behavior.

**BREAKING CHANGE**: Built-in metrics are now enabled by default. Users
who previously did not set any environment variables will have metrics
collection and export turned on automatically after upgrading. To
restore the previous behavior and disable metrics, thry have to set the
`SPANNER_DISABLE_BUILTIN_METRICS` environment variable to `true`.

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) ([64aebe7e](https://togithub.com/googleapis/python-spanner/commit/64aebe7e))

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

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

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

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

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

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

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

### Bug Fixes

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

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

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

### Documentation

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

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

</details>

#1459)

* chore: Remote most unit tests, lint, doc testing from kokoro presubmit

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Update sync-repo-settings.yaml

* support cryptograph>=3.1 (#1454)

* use presubmit config rather than nox.options.session

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* revert to using nox.options.sessions

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Shuowei Li <shuowei@google.com>