metadata: stabilize ValueFromIncomingContext

[KarthikReddyPuli]

ValueFromIncomingContext is an API available in metadata package which can be used to fetch single key from grpc metadata with lower memory footprint and cpu time. This PR stabilises the API after which this can be used safely

Fixes: #7298

RELEASE NOTES:

• metadata: stabilize ValueFromIncomingContext

[linux-foundation-easycla]

• ✅login: KarthikReddyPuli / (f8a2089)

The committers listed above are authorized under a signed CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.47%. Comparing base (f199062) to head (f8a2089).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7368      +/-   ##
==========================================
- Coverage   81.64%   81.47%   -0.17%     
==========================================
  Files         348      348              
  Lines       26755    26755              
==========================================
- Hits        21844    21799      -45     
- Misses       3744     3769      +25     
- Partials     1167     1187      +20     

Files	Coverage Δ
metadata/metadata.go	89.09% <ø> (ø)

... and 23 files with indirect coverage changes