Add max_client_conn metric

[JordanP]

What does this PR do?

Add a new PGBouncer metric: max_client_conn

Motivation

So we can compute a percent of client conn used:
(cl_active + cl_waiting) / max_client_conn

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

[JordanP]

@blaines @FlorianVeaux @hithwen fyi :)

[codecov]

Codecov Report

Merging #10033 (8b5d684) into master (5071fac) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Flag	Coverage Δ
pgbouncer	90.45% <100.00%> (+0.18%)	⬆️

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

[JordanP]

@coignetp thanks for adding the appropriate labels.

It looks like a CI check failed, but I don't know why, error is:

Run DataDog/labeler@glob-all
Error: Error: Input required and not supplied: repo-token
Error: Input required and not supplied: repo-token

Looks like some github action may be missing some config.

[coignetp]

Thank you for opening this PR! I just have a suggestion for readability

[coignetp]

@coignetp thanks for adding the appropriate labels.

It looks like a CI check failed, but I don't know why, error is:

Run DataDog/labeler@glob-all
Error: Error: Input required and not supplied: repo-token
Error: Input required and not supplied: repo-token

Looks like some github action may be missing some config.

It's expected, external contributors can't run this action, I applied the labels manually

[coignetp]

@JordanP FYI this was was released in the integration version 3.1.0. It should be shipped in version 7.32.0 of the datadog agent, but you can already manually install it if needed following this guide https://www.datadoghq.com/blog/agent-integration-management/

Package name is datadog-pgbouncer==3.1.0

[JordanP]

Excellent @coignetp ! Thank you so much !