Perf benchmark tests for span creation ( along with context management )

[lalitb]

Changes

As we are discussing performance changes for the api, it's important to get some numbers for span creation and context management involved in the process. The tests are:

• Test to measure performance for single-span creation
• Test to measure performance for single-span creation with scope ( involves stack )
• Test to measure performance for nested span creation with scope
• Test to measure performance for nested span creation with manual span context management (no stack involved)
• Test to measure performance for nested span creation with context propagation

These are the numbers as tested locally:

------------------------------------------------------------------------------------------
Benchmark                                                Time             CPU   Iterations
------------------------------------------------------------------------------------------
BM_SpanCreation                                        263 ns          263 ns      2538457
BM_SpanCreationWithScope                              2222 ns         2221 ns       314466
BM_NestedSpanCreationWithScope                        7892 ns         7891 ns        87182
BM_SpanCreationWithManualSpanContextPropagation       1164 ns         1164 ns       629233
BM_SpanCreationWitContextPropagation                  6642 ns         6641 ns        97745

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

Merging #856 (6937a54) into main (8a8ffe5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #856   +/-   ##
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  Coverage   95.50%   95.50%           
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  Files         156      156           
  Lines        6620     6620           
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  Hits         6322     6322           
  Misses        298      298