Fix and tests for multiple metric readers

[hannahhaering]

Fixes #2938

Changes

• Added tests as suggested in the issue
• Fix: the spec states "The SDK MUST NOT allow a MetricReader instance to be registered on more than one MeterProvider instance". Added a test for this and noticed that the test failed, so I added a verification in the MetricReader.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[martincostello]

Can you also update the CHANGELOG please, as this changes behaviour for users.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.74%. Comparing base (5852193) to head (1330c5f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6458      +/-   ##
==========================================
+ Coverage   86.57%   86.74%   +0.17%     
==========================================
  Files         258      258              
  Lines       11876    11878       +2     
==========================================
+ Hits        10282    10304      +22     
+ Misses       1594     1574      -20     

Flag	Coverage Δ
unittests-Project-Experimental	86.40% <100.00%> (+0.11%)	⬆️
unittests-Project-Stable	86.64% <100.00%> (+0.59%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	88.03% <100.00%> (+0.20%)	⬆️

... and 6 files with indirect coverage changes