Update to the metric part

[SamerNaffah]

What does this PR do?

I added more information, that we only collect what is exposed by linkerd, since it is confusing some customers. I also added that all the metrics ending with "_total" are appended by "count" by datadog : https://github.com/DataDog/integrations-core/blob/7585dad4f622339f87eed87ad15e2986d9025bde/linkerd/datadog_checks/linkerd/metrics.py#L123-L125 , since it can also lead to confusion.

Motivation

https://datadog.zendesk.com/agent/tickets/835258

Additional Notes

no

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

[buraizu]

Thanks, created DOCS-3738 to track this update