support OTEL_EXPORTER_OTLP_PROTOCOL

[tomDev5]

Fixes #1983

Changes

Added build_default for MetricExporter, LogExporter and SpanExporter.

build_default uses the OTEL_EXPORTER_OTLP_PROTOCOL environment variable (specified in opentelemetry docs).

This doesn't prevent creating them the old way, with all configurations possible.

[linux-foundation-easycla]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 39 lines in your changes missing coverage. Please review.

Project coverage is 81.2%. Comparing base (8499914) to head (0fd3265).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/logs.rs	0.0%	13 Missing ⚠️
opentelemetry-otlp/src/metric.rs	0.0%	13 Missing ⚠️
opentelemetry-otlp/src/span.rs	0.0%	13 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2977     +/-   ##
=======================================
- Coverage   81.3%   81.2%   -0.2%     
=======================================
  Files        126     126             
  Lines      24375   24414     +39     
=======================================
+ Hits       19827   19828      +1     
- Misses      4548    4586     +38     

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