[CODE HEALTH] Fix cppcoreguidelines-pro-type-const-cast for multi observer metrics.

[marcalff]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

• Resolved cppcoreguidelines-pro-type-const-cast warnings in the multi observer code.
• Changed the observer result map to keep const instruments instead of non const objects.
• Adjusted the code accordingly.

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

[marcalff]

@KJTsanaktsidis

Hi. Could you review this change when you have a chance ?

Small cleanup on the observer_results_ map to use const pointers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.25%. Comparing base (58ed80d) to head (9965fa4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4499      +/-   ##
==========================================
+ Coverage   83.24%   83.25%   +0.01%     
==========================================
  Files         521      521              
  Lines       20384    20381       -3     
==========================================
- Hits        16967    16966       -1     
+ Misses       3417     3415       -2     

Files with missing lines	Coverage Δ
sdk/src/metrics/async_instruments.cc	100.00% <100.00%> (ø)
sdk/src/metrics/multi_observer_result.cc	92.31% <100.00%> (-0.41%)	⬇️
sdk/src/metrics/state/observable_registry.cc	97.62% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[dbarker]

Looks good. Thanks