do not use MultiSpanProcessor for single processors

[Hailios]

Fixes # (issue)

Changes

do not wrap single processors in a multi span processor. that is, use it when needed, otherwise not. idea here is to save execution time by not needlessly going through the multi span processor.

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]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.53%. Comparing base (0668d64) to head (00e2150).

Files with missing lines	Patch %	Lines
sdk/src/trace/tracer_context.cc	55.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4043      +/-   ##
==========================================
- Coverage   82.04%   81.53%   -0.50%     
==========================================
  Files         385      385              
  Lines       15895    15909      +14     
==========================================
- Hits        13039    12970      -69     
- Misses       2856     2939      +83     

Files with missing lines	Coverage Δ
...k/include/opentelemetry/sdk/trace/tracer_context.h	100.00% <ø> (ø)
sdk/src/trace/tracer_context.cc	77.50% <55.00%> (-7.11%)	⬇️

... and 2 files with indirect coverage changes

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