[CMAKE] Add CMake script to find or fetch prometheus-cpp

[dbarker]

Fixes # (issue)

Changes

• Adds CMake script to find or fetch prometheus-cpp
• Explicitly link to the core and pull targets from the prometheus exporter.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	7845429
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (a1e6011) to head (7845429).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3522   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         219      219           
  Lines        7051     7051           
=======================================
  Hits         6343     6343           
  Misses        708      708           

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[owent]

LGTM

[marcalff]

LGTM