Add MeterContext::ForEachMeter() method to process callbacks on Meter in thread-safe manner

[lalitb]

Fixes #1720

Changes

Another approach to fix #1720 (refer #1777 for first approach) by adding MeterContext::ForEachMeter, which invokes callback to process each meter in thread-safe manner. This will ensure that metric collection doesn't crash for existing meters while new meter is being added.

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

[johanpel]

Thanks @lalitb , this looks like a more performant solution 👍

[lalitb]

CI failing with transient error for now -
E: Failed to fetch https://packages.microsoft.com/ubuntu/20.04/prod/dists/focal/main/binary-arm64/Packages.gz File has unexpected size (28549 != 28637). Mirror sync in progress? [IP: 13.93.[224](https://github.com/open-telemetry/opentelemetry-cpp/actions/runs/3485953430/jobs/5832023148#step:4:225).173 443] Hashes of expected file:

[codecov]

Codecov Report

Merging #1783 (bd54556) into main (d0571d8) will decrease coverage by 0.01%.
The diff coverage is 88.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1783      +/-   ##
==========================================
- Coverage   85.79%   85.79%   -0.00%     
==========================================
  Files         171      171              
  Lines        5214     5220       +6     
==========================================
+ Hits         4473     4478       +5     
- Misses        741      742       +1     

Impacted Files	Coverage Δ
sdk/src/metrics/meter_context.cc	88.89% <85.72%> (-0.76%)	⬇️
sdk/src/metrics/state/metric_collector.cc	92.31% <100.00%> (+0.31%)	⬆️

[marcalff]

LGTM