[SDK] Metrics ObservableRegistry Cleanup

[perhapsmaple]

Fixes #2373, #2081

Changes

Added a function called CleanupCallbacks to ObservableRegistry that is called from the destructor of ObservableInstrument to remove back pointers from the registry.

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

[lalitb]

Thanks.

[codecov]

Codecov Report

Merging #2376 (55cb77d) into main (e918960) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2376      +/-   ##
==========================================
+ Coverage   87.41%   87.43%   +0.03%     
==========================================
  Files         199      199              
  Lines        6018     6028      +10     
==========================================
+ Hits         5260     5270      +10     
  Misses        758      758              

Files	Coverage Δ
sdk/src/metrics/async_instruments.cc	100.00% <100.00%> (ø)
sdk/src/metrics/state/observable_registry.cc	83.34% <100.00%> (+3.34%)	⬆️

[marcalff]

LGTM, thanks for the fix.