Fall back to system_health/event_file when querying deadlocks

[nenadnoveljic]

What does this PR do?

If the datadog XE session isn't created, the agent will retrieve deadlocks from system_health/event_file.

Motivation

Some customers can't or prefer not to create dedicated XE sessions. We use event_file over ring_buffer to avoid truncating recent events, but datadog XE session remains the preferred option for better performance.

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

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.77%. Comparing base (14c88c4) to head (1fc1079).
Report is 4 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?
sqlserver	91.15% <88.00%> (+9.50%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[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.

[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.