feat: add support for OTEL_EXPORTER_OTLP_PROTOCOL, OTEL_EXPORTER_OTLP_ENDPOINT

[seemk]

• Drop dubious possible values for OTEL_TRACES_EXPORTER, e.g. otlp-splunk which meant the HTTP protobuf exporter.
• Switching between gRPC and HTTP is now done via OTEL_EXPORTER_OTLP_PROTOCOL and OTEL_EXPORTER_OTLP_TRACES_PROTOCOL with the latter taking precedence as per OTel spec.
• Add support for OTEL_EXPORTER_OTLP_TRACES_ENDPOINT. Order of precedence: programmatically -> OTEL_EXPORTER_OTLP_TRACES_ENDPOINT -> OTEL_EXPORTER_OTLP_ENDPOINT.
• It is now possible to specify multiple exporters, separated by commas to OTEL_TRACES_EXPORTER. The only accepted values for now are otlp and console.

[codecov-commenter]

Codecov Report

Base: 82.28% // Head: 83.68% // Increases project coverage by +1.39% 🎉

Coverage data is based on head (37135fa) compared to base (05cc17c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #599      +/-   ##
==========================================
+ Coverage   82.28%   83.68%   +1.39%     
==========================================
  Files          28       28              
  Lines        1118     1140      +22     
  Branches      248      255       +7     
==========================================
+ Hits          920      954      +34     
+ Misses        198      186      -12     

Impacted Files	Coverage Δ
src/tracing/options.ts	99.21% <100.00%> (+10.23%)	⬆️
src/utils.ts	95.83% <100.00%> (+1.38%)	⬆️

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