allow extension of the lifetime of ContextStorage.

[esigo]

Fixes #1211 (issue)

Changes

The specifications require the context to be immutable, but does not talk about whether the users are allowed to have access to the storage.
The change in this PR allows the users to get a shared_ptr to const so they can't do anything with the pointer but extending the lifetime of the storage.

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 #1214 (bfbf9f4) into main (1026ec3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1214   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files         196      196           
  Lines        7038     7038           
=======================================
  Hits         6544     6544           
  Misses        494      494           

Impacted Files	Coverage Δ
...pi/include/opentelemetry/context/runtime_context.h	97.50% <ø> (ø)

[ThomsonTan]

Could we provide some dedicated helper method to increase the lifetime of RuntimeStorage? As the user cannot deduce the purpose of this API directly from the API name.

[esigo]

Could we provide some dedicated helper method to increase the lifetime of RuntimeStorage? As the user cannot deduce the purpose of this API directly from the API name.

Could you please elaborate?

[ThomsonTan]

Could you please elaborate?

I meant some explicit API like AddRef/ReleaseRef but this seems requiring 2 APIs. The current one looks good to through.