support OS environment variables for setting tracer to OTLP

[tsloughter]

Right now we have to throw out jaeger and zipkin exporters. I suppose we could do the same as is being done here for otlp, simply set the module to what it should be, like opentelemetry_zipkin and the user is expected to have included the application...?

Starting to wonder if the exporter shouldn't just be a part of the SDK. I think it would be an easy call on OTP-24 since it'll support optional dependencies in .app but it currently would force inclusion of grpcbox and inets into every release.

[codecov]

Codecov Report

Merging #207 (06c0f40) into main (441d2af) will increase coverage by 0.23%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   34.46%   34.70%   +0.23%     
==========================================
  Files          64       64              
  Lines        3296     3308      +12     
==========================================
+ Hits         1136     1148      +12     
  Misses       2160     2160              

Flag	Coverage Δ
api	51.61% <ø> (ø)
elixir	14.03% <ø> (ø)
erlang	34.73% <78.57%> (+0.24%)	⬆️
exporter	18.54% <ø> (+0.04%)	⬆️
sdk	70.72% <78.57%> (+0.34%)	⬆️

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Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_configuration.erl	77.94% <72.72%> (-1.72%)	⬇️
apps/opentelemetry/src/opentelemetry_sup.erl	100.00% <100.00%> (ø)
...ntelemetry_exporter/src/opentelemetry_exporter.erl	72.81% <0.00%> (+0.97%)	⬆️
apps/opentelemetry/src/otel_batch_processor.erl	73.86% <0.00%> (+1.13%)	⬆️
apps/opentelemetry/src/otel_metric_accumulator.erl	79.16% <0.00%> (+2.08%)	⬆️

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