[WIP] Emit metrics for new headers handling behaviour

[biosvs]

Replaced with #2267

[codecov]

Codecov Report

Attention: Patch coverage is 80.80000% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 85.16%. Comparing base (407d76c) to head (ae5acc0).

Files	Patch %	Lines
transport/tchannel/response_writer.go	85.91%	5 Missing and 5 partials ⚠️
transport/http/outbound.go	42.85%	4 Missing and 4 partials ⚠️
transport/grpc/outbound.go	33.33%	3 Missing and 3 partials ⚠️
transport/http/handler.go	60.00%	3 Missing and 3 partials ⚠️
transport/tchannel/outbound.go	40.00%	4 Missing and 2 partials ⚠️
transport/grpc/handler.go	20.00%	2 Missing and 2 partials ⚠️
transport/grpc/headers.go	90.00%	2 Missing and 2 partials ⚠️
transport/tchannel/channel_outbound.go	0.00%	1 Missing and 1 partial ⚠️
transport/tchannel/handler.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2259      +/-   ##
==========================================
- Coverage   85.20%   85.16%   -0.04%     
==========================================
  Files         270      272       +2     
  Lines       15555    15687     +132     
==========================================
+ Hits        13253    13360     +107     
- Misses       1877     1890      +13     
- Partials      425      437      +12     

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