update and add exporter options

[tsloughter]

Added a couple new environment variables for configuring the exporter, from https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#configuration-options

Also updates the port for http_protobuf to the default used in the specification.

[codecov]

Codecov Report

Merging #341 (e16ffd3) into main (a22fb0e) will increase coverage by 16.89%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main     #341       +/-   ##
===========================================
+ Coverage   16.44%   33.33%   +16.89%     
===========================================
  Files          21       56       +35     
  Lines         596     4563     +3967     
===========================================
+ Hits           98     1521     +1423     
- Misses        498     3042     +2544     

Flag	Coverage Δ
api	67.28% <ø> (+50.83%)	⬆️
elixir	16.44% <ø> (ø)
erlang	33.27% <100.00%> (?)
exporter	21.47% <100.00%> (?)
sdk	77.13% <ø> (?)
zipkin	2.59% <ø> (?)

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	74.09% <100.00%> (ø)
apps/opentelemetry/src/otel_resource_env_var.erl	87.50% <0.00%> (ø)
apps/opentelemetry/src/otel_span_sweeper.erl	70.83% <0.00%> (ø)
apps/opentelemetry/src/otel_links.erl	73.33% <0.00%> (ø)
apps/opentelemetry/src/otel_span_utils.erl	90.62% <0.00%> (ø)
.../opentelemetry_zipkin/src/opentelemetry_zipkin.erl	38.98% <0.00%> (ø)
apps/opentelemetry/src/otel_tracer_default.erl	100.00% <0.00%> (ø)
...elemetry/src/otel_sampler_trace_id_ratio_based.erl	71.42% <0.00%> (ø)
apps/opentelemetry/src/otel_exporter_pid.erl	80.00% <0.00%> (ø)
...metry_exporter/src/opentelemetry_trace_service.erl	25.00% <0.00%> (ø)
... and 41 more

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