Prevent hostname evaluating to None in sqlserver check

[jmeunier28]

What does this PR do?

This change prevents a very bad bug where the host field on metrics and events will fall back to None when the static information cache reaches its TTL of 1 day. When this happens, we see gaps in data for sqlserver instances because our backend drops these as bad payloads. We can see an example of this happening in our integration environment:

... meanwhile, if we log the payloads we see the host value is set to null:

{"host":null,"ddagentversion":"7.56.0","ddsource":"sqlserver",...

Motivation

https://datadoghq.atlassian.net/browse/DBMON-4711

In this previous change #17750 we updated the check to call

    def check(self, _):
        if self.do_check:
            self.load_static_information()

on each check run. This function does a few things:

1. Checks to see we are missing key/value pairs in our static information cache
2. re-loads the cache if this is the case
3. if keys are missing, it also resets the self._resolved_hostname value to None, but it never initializes the value

This makes the load_static_information not safe to call outside of the context of the set_resolved_hostname function. In order to prevent the resolved_hostname property from ever returning None we add a null check && re-initialize the value if necessary:

        if self._resolved_hostname is None:
             self.set_resolved_hostname()
        return self._resolved_hostname

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• 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

[ofek]

After merging, can we please add a test?

[jmeunier28]

merged this one instead #18249 so i can deal with the tests separately. I cannot easily run the tests / fix them tonight and this is blocking the agent release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (master@2b35698). Learn more about missing BASE report.
Report is 10 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
sqlserver	91.08% <100.00%> (?)

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

[nenadnoveljic]

Can you link to the Jira card?