Fix race conditions when running many instances of the Agent

[justiniso]

What does this PR do?

This removes access to mutable global vars.

Motivation

This is being reported as a bug by customers who are seeing

self._submit_metrics(metrics, results, tags)\\n  File \\\"/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/mysql/mysql.py\\\", line 615, in _submit_metrics\\n    for variable, metric in iteritems(variables):\\nRuntimeError: dictionary changed size during iteration\\n\"}]","check":"mysql"}

in the check logs. Basically what is happening is:

1. We set the metrics global var equal to STATUS_VARS, which is a mutable dict

2. This is updated at various points in this function

3. Sometimes these updates are conditional

4. ... And is iterated over here

5. There seems to be some kind of race with another instance of the integration? As long as the code path followed every time is the same, no harm, no foul but that isn’t happening here. Making a copy of the dict before updating will prevent this possibility.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[jmeunier28]

weird looks like these should have been capitalized all along https://github.com/DataDog/integrations-core/blob/master/mysql/datadog_checks/mysql/const.py#L218 ?

[justiniso]

Yeah I'm trying to figure out why this change is affecting the metrics; that's not clear to me yet, but these values are definitely wrong.

[codecov]

Codecov Report

Merging #12342 (6413a32) into master (8c2d643) will increase coverage by 0.04%.
The diff coverage is 100.00%.

Flag	Coverage Δ
mysql	89.01% <100.00%> (+1.49%)	⬆️

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