Cleanup of context from Runtime storage

[lalitb]

Fixes #884

Changes

ThreadLocalContextStorage is used as default runtime storage, and is internally implemented as a Stack. The Pop() operation doesn't perform the cleanup of stored Context object, but only decrements the stack size. The actual cleanup happens only when that storage is later re-utilized with a new Context. The side effect of this approach is that any shared_ptr object stored in the popped-out Context will not be cleaned up at the time of Pop() operation.

The fix is to replace the current context (meant to be popped out) with an empty Context object before decrementing the size.
This will release the ownership of any shared_ptr object stored in this current context.

Also, Stack::Pop() shouldn't be returning the removed object, to be consistent with std::stack api.

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 #885 (162e05a) into main (51b6b19) will increase coverage by 0.01%.
The diff coverage is 93.34%.

@@            Coverage Diff             @@
##             main     #885      +/-   ##
==========================================
+ Coverage   95.40%   95.40%   +0.01%     
==========================================
  Files         158      158              
  Lines        6705     6717      +12     
==========================================
+ Hits         6396     6408      +12     
  Misses        309      309              

Impacted Files	Coverage Δ
...pi/include/opentelemetry/context/runtime_context.h	97.47% <66.67%> (ø)
sdk/test/trace/tracer_test.cc	99.48% <100.00%> (+0.02%)	⬆️

[ThomsonTan]

How could someone get a shared_ptr of the Context stored in ThreadLocalContextStorage?

[lalitb]

Line 237 is actually resetting the base[size - 1] with empty Context() object. This is to ensure that previously-stored shared_ptr<Span> gets released. The new empty Context() is not supposed to be used again.

[lalitb]

@maxgolov - Would like to include these changes in the next release. Would it be good to merge, as you wanted to do some multi-threaded tests with these changes?

[lalitb]

@maxgolov Merging this. If there are issues found in your testing, we can address them separately. Thanks.