Clarify the enabled property in database_metrics configuration descriptions for sqlserver

[azhou-datadog]

What does this PR do?

Add the enabled property to description of database_metrics configuration descriptions for sqlserver

Motivation

Realized thanks to help from @willstrickfaden and Faizan Hussain in the support channel, that the conf.yaml.example does not sufficiently surface the properties available for the database_metrics configurations. Most critically, it is not clear that in order to enable an metric, the format must look something like this (with the enabled property set to true):

...
database_metrics:
  master_files_metrics:
    enabled: true
  index_usage_metrics:
    enabled: true
  db_fragmentation_metrics:
    enabled: true
...

This PR explicitly states each of the properties available for each database_metric configuration, making this more clear.

Please note, the include_xyz_metrics: true configuration format will still continue to work in future agent releases as well, in order to maintain backwards compatibility of conf.yaml files.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[github-actions]

The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.98%. Comparing base (32f8ae9) to head (1e3bbd5).
Report is 4 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
sqlserver	91.15% <ø> (+9.51%)	⬆️

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