Add support for OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS and OTEL_EXPORTER_OTLP_TIMEOUT env vars

[dszmigielski]

Partially addresses #1453.

This feature is important so that we can reuse the SDK in https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation.

Changes

The OtlpExporterOptions defaults can be overridden using OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS and OTEL_EXPORTER_OTLP_TIMEOUT environmental variables as defined in the specification.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2188 (f38f4b0) into main (89a0bce) will increase coverage by 0.00%.
The diff coverage is 76.92%.

@@           Coverage Diff           @@
##             main    #2188   +/-   ##
=======================================
  Coverage   74.65%   74.66%           
=======================================
  Files         217      217           
  Lines        6948     6974   +26     
=======================================
+ Hits         5187     5207   +20     
- Misses       1761     1767    +6     

Impacted Files	Coverage Δ
...tation/OpenTelemetryProtocolExporterEventSource.cs	55.55% <57.14%> (+0.55%)	⬆️
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	88.46% <84.21%> (-11.54%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (+2.94%)	⬆️