add support for setting grpc metadata

[ktalg]

Closes #663

[codecov]

Codecov Report

Base: 88.6% // Head: 88.7% // Increases project coverage by +0.1% 🎉

Coverage data is based on head (a1c9147) compared to base (a2dbeac).
Patch coverage: 100.0% of modified lines in pull request are covered.

❗ Current head a1c9147 differs from pull request most recent head 58cfdd8. Consider uploading reports for the commit 58cfdd8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #671     +/-   ##
========================================
+ Coverage    88.6%   88.7%   +0.1%     
========================================
  Files          30      30             
  Lines        4368    4395     +27     
========================================
+ Hits         3871    3898     +27     
  Misses        331     331             
  Partials      166     166             

Impacted Files	Coverage Δ
fhttp/http_client.go	84.3% <ø> (ø)
fgrpc/grpcrunner.go	90.7% <100.0%> (+1.2%)	⬆️
fgrpc/pingsrv.go	89.8% <100.0%> (+0.6%)	⬆️

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[ldemailly]

that user-agent thing sort of bothers me but we can figure it out later