Add user connections metric

[alexbarksdale]

What does this PR do?

Adds a new metric mysql.performance.user_connections as a follow up to the reversion of a metric type (PR).

Initially, the migrated metric was used to surface user connections, but because this metric (mysql.net.connections) had a different meaning:

The rate of connections to the server

another metric has to replace it. There is an existing metric mysql.performance.threads_connected that seemed to be a good candidate, however, this metric represents all open connections which includes background threads which is not what we need. We need a metric to surface the number of user connections.

Motivation

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 #12087 (bee7dc9) into master (46b4923) will increase coverage by 0.04%. The diff coverage is 84.21%.

Flag	Coverage Δ
mysql	88.84% <84.21%> (+1.32%)	:arrow_up:

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