Update Metrics status in README.md

[lalitb]

Changes

• Tag Metrics as stable in README.md
• Remove section "OpenTelemetry Specification Compatibility Matrix". This was supposed to map the compatibility between specs version and otel-cpp version, but not much use is coming out of it.

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

Merging #1722 (e570c7e) into main (9acde87) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1722   +/-   ##
=======================================
  Coverage   85.79%   85.79%           
=======================================
  Files         171      171           
  Lines        5212     5212           
=======================================
  Hits         4471     4471           
  Misses        741      741