[EXPORTERS] Remove prometheus timestamps

[carlosroman]

Fixes #2316

Changes

This returns the previous behavior from #2324 that was removed in #2213.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (9b983a2) to head (2253c8e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3895   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files         226      226           
  Lines        7200     7200           
=======================================
  Hits         6483     6483           
  Misses        717      717           

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