exporter: support for endpoint and header configuration

[tsloughter]

This is quick and a little dirty but it adds the ability to add headers to export requests, needed for #202

I'll have another PR with support for the remaining exporter config options (certificate, compression and timeout).

[codecov]

Codecov Report

Merging #206 (62e5ba3) into main (abf1e6b) will increase coverage by 0.48%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##             main     #206      +/-   ##
==========================================
+ Coverage   34.06%   34.55%   +0.48%     
==========================================
  Files          64       53      -11     
  Lines        3264     3218      -46     
==========================================
  Hits         1112     1112              
+ Misses       2152     2106      -46     

Flag	Coverage Δ
api	55.32% <ø> (+3.70%)	⬆️
elixir	?
erlang	34.55% <78.94%> (+0.46%)	⬆️
exporter	18.49% <76.66%> (+0.60%)	⬆️
sdk	70.65% <87.50%> (+0.47%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ntelemetry_exporter/src/opentelemetry_exporter.erl	71.84% <76.66%> (-0.31%)	⬇️
apps/opentelemetry/src/otel_configuration.erl	79.66% <87.50%> (+1.22%)	⬆️
apps/opentelemetry_api/src/otel_baggage.erl	74.07% <0.00%> (-7.41%)	⬇️
apps/opentelemetry_api/src/otel_ctx.erl	75.00% <0.00%> (-4.17%)	⬇️
apps/opentelemetry_api/src/opentelemetry.erl	68.75% <0.00%> (-1.57%)	⬇️
...entelemetry_api/lib/open_telemetry/sum_observer.ex
apps/opentelemetry_api/lib/open_telemetry.ex
...etry_api/lib/open_telemetry/updown_sum_observer.ex
...ps/opentelemetry_api/lib/open_telemetry/counter.ex
...telemetry_api/lib/open_telemetry/updown_counter.ex
... and 8 more

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[bryannaegele]

Are we going to try to run everything through the new configuration module or will the exporter handle its own?

[tsloughter]

@bryannaegele a bit of both. When I first started this patch I was going with the exporter being configured through the SDK and thus otel_configuration but then changed to the configuration being part of the opentelemetry_exporter application, so otel_configuration is only used as a convenience function.