Add pool.latency_us metric and correct unit for pool.latency_ms

[ian28223]

What does this PR do?

• Correct unit for pool.latency_ms to report in milliseconds as per documentation.
• Introduce pool.latency_us which is actually in microsecond

Latency_ms is a ProxySQL typo which reported values in microseconds instead of milliseconds. This has then been renamed to Latency_us since ProxySQL 1.3.3

Motivation

AGENT-6978

Additional Notes

• https://github.com/DataDog/integrations-core/blob/7.31.x/proxysql/datadog_checks/proxysql/queries.py#L180-L181
• https://github.com/sysown/proxysql/blob/v2.x/doc/release_notes/ProxySQL_v1.3.3.md

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]

Codecov Report

Merging #10335 (ad7c61f) into master (feff48d) will not change coverage.
The diff coverage is n/a.

Flag	Coverage Δ
proxysql	98.97% <ø> (ø)

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[coignetp]

Why introducing a new latency_us ? It's duplicated from latency_ms

[ian28223]

Why introducing a new latency_us ? It's duplicated from latency_ms

Reason is that Latency_us is the actual metric since v1.3.3 (2017 release)

• https://proxysql.com/documentation/stats-statistics/#:~:text=fields%E2%80%99%20description%2C%20etc)-,Latency_us,-%3A%20the%20current%20ping

And I didn't want to remove latency_ms for backwards compatibility (in case some user already have this in their monitors) so I had it actually report in milliseconds as the name (ms) suggests.