[Metrics API] Simplify public API contract of instruments for SDK

[utpilla]

Changes

• All the synchronous instruments can utilize a single trait instead of having dedicated traits such as SyncCounter, SyncGauge, SyncHistogram etc.
• This PR introduces a single trait SyncInstrument which all the synchronous instruments can use
• This also reduces the number of public APIs exposed

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.0%. Comparing base (c5cfa40) to head (5aec607).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry/src/metrics/noop.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2207   +/-   ##
=====================================
  Coverage   79.0%   79.0%           
=====================================
  Files        122     122           
  Lines      21086   21056   -30     
=====================================
- Hits       16672   16651   -21     
+ Misses      4414    4405    -9     

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[cijothomas]

Nice!