[Azure Metrics] Fix CassandraConnectionClosures metric configuration

[zmoog]

What does this PR do?

Move the CassandraConnectionClosures metric name from the always-available list of metrics to the Cassandra-focused list of metrics.

Metric names not always available should be listed in a section marked with the ignore_unsupported setting set to true.

Why is it important?

The integration works if the subscription has at least one Cassandra cluster available but fails otherwise.

Checklist

- [ ] My code follows the style guidelines of this project
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation

• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Related [Azure Metrics] the integration fails with the "metric names configured CassandraConnectionClosures are not supported for the resource" error integrations#5453

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @zmoog? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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[zmoog]

/test

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b zmoog/azure-metrics-database-error upstream/zmoog/azure-metrics-database-error
git merge upstream/main
git push upstream zmoog/azure-metrics-database-error

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b zmoog/azure-metrics-database-error upstream/zmoog/azure-metrics-database-error
git merge upstream/main
git push upstream zmoog/azure-metrics-database-error