[Feat] Adding header metric in thrift handler for unsafe headers

[pulkit4tech]

• Description and context for reviewers: one partner, one stranger
  • Adding header metric in thrift handler for unsafe headers
    • setting meter in context when call start from observ middleware and used just before we encode and pass request to service handler
  • Testing - https://code.uberinternal.com/D21746817

Flow -

setting MetricInfo in Obs middleware and used in thrift handlers

RELEASE NOTES: Adding header metric in thrift handler for unsafe headers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.18%. Comparing base (25d2ad2) to head (b77b870).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2431      +/-   ##
==========================================
+ Coverage   85.13%   85.18%   +0.05%     
==========================================
  Files         279      280       +1     
  Lines       16611    16657      +46     
==========================================
+ Hits        14141    14189      +48     
+ Misses       2010     2009       -1     
+ Partials      460      459       -1     

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[pulkit4tech]

I'll remove this, was testing.