[WIP] Fix metrics context circular reference

[esigo]

trafficstars

Fixes #1534 (issue)

Changes

Please provide a brief description of the changes here.

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[bot]]

Codecov Report

Merging #1535 (a4052af) into main (86c9f2b) will decrease coverage by 0.57%. The diff coverage is 43.59%.

@@            Coverage Diff             @@
##             main    #1535      +/-   ##
==========================================
- Coverage   83.85%   83.28%   -0.56%     
==========================================
  Files         156      156              
  Lines        4908     4970      +62     
==========================================
+ Hits         4115     4139      +24     
- Misses        793      831      +38     

Impacted Files	Coverage Δ
sdk/src/metrics/meter.cc	4.80% <0.00%> (-0.46%)	:arrow_down:
sdk/src/metrics/state/observable_registry.cc	14.29% <0.00%> (-0.86%)	:arrow_down:
sdk/src/metrics/sync_instruments.cc	79.24% <55.77%> (-16.00%)	:arrow_down:
sdk/src/metrics/state/metric_collector.cc	48.00% <66.67%> (-2.00%)	:arrow_down:
sdk/src/metrics/meter_context.cc	42.86% <100.00%> (ø)

[lalitb]

@esigo - I have gone through the circular reference between meter_context <-> meter, and meter_context <-> metric_collector, and I don't think it would be easy to avoid them. These changes look good in that scenario. We can have a separate issue to track the redesign requirement for metrics v1.0 / GA.