Adding deprecated notices to old metrics

[nascowe]

A range of metrics (95percentile, avg, count, max, median) are deprecated since 2018 and confusing recent users. Customers are recommended to use the aggregation functions for queries in lieu: https://docs.datadoghq.com/dashboards/querying/#aggregate-and-rollup The statistical metrics are still relevant for those using agent v5 hence not removing the metrics from the list entirely.

What does this PR do?

Adds a deprecated note to the relevant statistical-related metrics that are not present in agent 6+.

Motivation

Customer enquired why they didn't have the metrics showing in their environment.

Additional Notes

ZD Ticket #149476 for evidence of deprecation.

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

[yzhan289]

@nascowe bump on Celene's comment, otherwise looks good on our side.