Add User-Agent header to exporter requests

[albertored]

Fixes #473

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a79a39f) 72.94% compared to head (0bf88c2) 73.02%.

Files	Patch %	Lines
...ntelemetry_exporter/src/opentelemetry_exporter.erl	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #605      +/-   ##
==========================================
+ Coverage   72.94%   73.02%   +0.08%     
==========================================
  Files          61       61              
  Lines        1918     1924       +6     
==========================================
+ Hits         1399     1405       +6     
  Misses        519      519              

Flag	Coverage Δ
api	69.64% <ø> (ø)
elixir	17.47% <ø> (ø)
erlang	74.35% <75.00%> (+0.08%)	⬆️
exporter	67.47% <75.00%> (+0.81%)	⬆️
sdk	78.69% <ø> (ø)
zipkin	54.16% <ø> (ø)

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[albertored]

@tsloughter moving into draft because I need to verify how to add the user agent also on grpc calls

[albertored]

@tsloughter we need to fix grpcbox for this to work correctly, see linked PR

[albertored]

@tsloughter with the recent bump of grpcbox this should be ready to be merged