Add the `max_connections` metric

[FlorentClarret]

What does this PR do?

Add the max_connections metric to the mcache integration.

Motivation

Resolves https://github.com/DataDog/integrations-core/issues/12684

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

[codecov[bot]]

Codecov Report

Merging #12689 (cfefad9) into master (9410e98) will decrease coverage by 0.00%. The diff coverage is 66.66%.

Flag	Coverage Δ
mcache	93.26% <66.66%> (-0.26%)	:arrow_down:

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[hithwen]

Can you add aggregator.assert_metrics_using_metadata(get_metadata_metrics() to the integration and end to end tests?