Return Scope object instead of unique_ptr<Scope> from Tracer::WithActiveSpan()

[lalitb]

Fixes #787

Changes

• As mentioned in why nostd::unique_ptr<Scope> not Scope #787 , Tracer::WithActiveSpan() can avoid memory allocation on heap by returning newly created Scope object instead of unique_ptr<Scope>. The copy constructor of Scope is implicitly deleted because of unique_ptr member variable, so this ensures multiple copies are not possible for Scope.

• Also modified etw::tracer::StartSpan() method to not make span active immediately after creating as this deviates from the specs. tracer::WithActiveSpan() should be called explicitly by instrumentation library to make it active. ( cc @maxgolov )

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 #788 (e29d3d2) into main (3fa6525) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #788      +/-   ##
==========================================
+ Coverage   95.99%   96.00%   +0.01%     
==========================================
  Files         176      176              
  Lines        7186     7183       -3     
==========================================
- Hits         6898     6896       -2     
+ Misses        288      287       -1     

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/tracer.h	100.00% <100.00%> (ø)
api/test/trace/tracer_test.cc	100.00% <100.00%> (ø)
...include/opentelemetry/sdk/common/circular_buffer.h	100.00% <0.00%> (+2.12%)	⬆️

[jsuereth]

We may want to have a secondary method to apply active-span to context that DOESN'T have auto-scope (for advanced use cases).

Like this cleanup!