[SDK] support aggregation of identical instruments

[dbarker]

Fixes #3356

Why?
The spec requires identical instruments to be aggregated and duplicate instruments to be detected and exported.

• Instruments are identical if all identifying fields equal: case-insensitive name, kind, unit, description
• Instruments are duplicates if the case-insensitive names match but another field is different.

Current behavior:

• Instrument names are treated as case sensitive and the only identifier used to track instruments by meter.
• Identical instruments are not aggregated.
  • If a user creates another instrument with the same name then the first instrument's data is no longer collected/exported.
• Duplicate instruments are not detected and reported to the user

New behavior:

• Instruments are identified based on all identifying fields.
• Names are treated as case-insensitive per the spec and name case conflicts result in the first-seen instrument by name being used when all other fields equal.
• Identical instruments are aggregated
• Duplicate instruments are detected and reported to the user. Duplicates are functional instruments (will export data) per the spec.

Changes

• checks if an instrument already has a storage object in the meter's registry before creating a new one.
• add tests for the spec requirements on aggregation of identical instruments
• add tests and log messages following the spec on Duplicate instrument registration.

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

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[codecov]

Codecov Report

Attention: Patch coverage is 85.36585% with 18 lines in your changes missing coverage. Please review.

Project coverage is 90.05%. Comparing base (4e4d8de) to head (53b98c5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...dk/include/opentelemetry/sdk/metrics/instruments.h	75.76%	16 Missing ⚠️
sdk/src/metrics/meter.cc	96.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3358      +/-   ##
==========================================
+ Coverage   89.98%   90.05%   +0.08%     
==========================================
  Files         211      212       +1     
  Lines        6812     6932     +120     
==========================================
+ Hits         6129     6242     +113     
- Misses        683      690       +7     

Files with missing lines	Coverage Δ
sdk/include/opentelemetry/sdk/metrics/meter.h	57.15% <ø> (ø)
sdk/src/metrics/meter.cc	85.89% <96.50%> (+3.39%)	⬆️
...dk/include/opentelemetry/sdk/metrics/instruments.h	75.76% <75.76%> (ø)

... and 3 files with indirect coverage changes

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[dbarker]

Hi @lalitb, Could you take a look at this PR and share your feedback? Thanks!

[lalitb]

Hi @lalitb, Could you take a look at this PR and share your feedback? Thanks!

Apologies for the delay—will review and share feedback in the next couple of days.

[lalitb]

@dbarker Thank you for your PR. This is very nicely done, thoroughly covers all test scenarios :)