chore: Add warning on Prometheus crate

[cijothomas]

Fixes #2451
As discussed in the community call, updating OTLP doc to show prometheus integration, and add warning to prometheus doc about discontinuation.

We can do 0.29 release and end it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.7%. Comparing base (969bedf) to head (c6d30a6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2831   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        124     124           
  Lines      23490   23490           
=====================================
  Hits       18961   18961           
  Misses      4529    4529           

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[lalitb]

Should we also mark it as deprecated in Cargo.toml as done by async-std -
https://github.com/async-rs/async-std/blob/96f564672a8588ac0516e86580c249009ad3e94f/Cargo.toml#L15

[cijothomas]

@scottgerring It looks like the benchmark action is triggered for this, even though the GitHub label is not added and this is not main!