[sdk] SuppressInstrumentationScope perf tweaks

[CodeBlanch]

Changes

Tweaked the implementation of SuppressInstrumentationScope to achieve better perf.

Before...

• We stored an int in the context slot. That int had to be boxed when scopes were created and we boxed a 0 when scopes were disposed.
• Any time we incremented or decremented in reference counting mode we pushed a new int to the context slot which had to be boxed AND creates a new ExecutionContext.

After...

• We store the SuppressInstrumentationScope instance itself in the context slot. That avoids the boxing and gives us storage for the reference count which we can increment/decrement freely without creating any new ExecutionContexts.

Benchmarks

Before:

Method	Mean	Error	StdDev	Gen0	Allocated
Begin	143.91 ns	2.027 ns	1.896 ns	0.0515	648 B
Enter	99.49 ns	1.226 ns	1.147 ns	0.0305	384 B

After:

Method	Mean	Error	StdDev	Gen0	Allocated
Begin	125.37 ns	1.738 ns	1.626 ns	0.0362	456 B
Enter	59.17 ns	0.710 ns	0.664 ns	0.0082	104 B

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Unit tests added/updated

[codecov]

Codecov Report

Merging #4304 (2c6d0e2) into main (7638d85) will decrease coverage by 0.07%.
The diff coverage is 88.57%.

❗ Current head 2c6d0e2 differs from pull request most recent head bff3687. Consider uploading reports for the commit bff3687 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4304      +/-   ##
==========================================
- Coverage   84.45%   84.39%   -0.07%     
==========================================
  Files         298      298              
  Lines       11854    11872      +18     
==========================================
+ Hits        10011    10019       +8     
- Misses       1843     1853      +10     

Impacted Files	Coverage Δ
src/OpenTelemetry/SuppressInstrumentationScope.cs	90.24% <88.57%> (-9.76%)	⬇️

... and 8 files with indirect coverage changes