Support OpenTelemetry environment variables?

[plantfansam]

OpenTelemetry supports a variety of environment variables. @yangxikun are you open to supporting those in this library? Users would need to include env directives in their NGINX configurations, but I think that's OK.

[yangxikun]

If you need this feature, welcome PR, thanks.

[plantfansam]

As a first pass, I think we can just do General SDK Configuration, Batch Span Processor, and Attribute limits:

• OTEL_RESOURCE_ATTRIBUTES
• OTEL_SERVICE_NAME
• OTEL_LOG_LEVEL
• OTEL_PROPAGATORS
• OTEL_TRACES_SAMPLER
• OTEL_TRACES_SAMPLER_ARG
• OTEL_BSP_SCHEDULE_DELAY
• OTEL_BSP_EXPORT_TIMEOUT
• OTEL_BSP_MAX_QUEUE_SIZE
• OTEL_BSP_MAX_EXPORT_BATCH_SIZE
• OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT
• OTEL_ATTRIBUTE_COUNT_LIMIT
• OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT
• OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT
• OTEL_SPAN_EVENT_COUNT_LIMIT    
• OTEL_EVENT_ATTRIBUTE_COUNT_LIMIT 
  Since the library doesn't yet support jaeger, zipkin, or prometheus exporter, we do not yet need to support env vars for those.

We also will not support various link env vars yet (OTEL_SPAN_LINK_COUNT_LIMIT, OTEL_LINK_ATTRIBUTE_COUNT_LIMIT).

[yangxikun]

OTEL*_LINK_* link is not implement yet. Others is ok.

[plantfansam]

OTEL*_LINK_* link is not implement yet. Others is ok.

👍 - I updated the comment.