[kafka-check] add under min ISR partition count metric

[OuesFa]

What does this PR do?

Add a metric about the number of partitions whose in-sync replicas count is less than minIsr.

Motivation

We'd need to monitor that metric to be alerted when some producers with acks=all config are not able to produce to a given topic.

Additional Notes

Since the cardinality is pretty low, it shouldn't be a problem regarding the number of metrics collected by the check.

Review checklist (to be filled by reviewers)

• 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