Support configuration of the SDK with environment variables

[MrAlias]

The specification states:

SDKs MAY choose to allow configuration via the environment variables in this specification, but are not required to. If they do, they SHOULD use the names listed in this document.

Currently we only support the OTEL_RESOURCE_ATTRIBUTES environment variable and have plans to support the OTLP exporter configuration via environment variables.

To accelerate the release of a stable 1.0.0 release we have postponed implementing this. After this release we need to implement the specified environment variables to support users requiring this.

• OTEL_RESOURCE_ATTRIBUTES
• 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_SPAN_ATTRIBUTE_COUNT_LIMIT
• OTEL_SPAN_EVENT_COUNT_LIMIT
• OTEL_SPAN_LINK_COUNT_LIMIT
• OTEL_EXPORTER_JAEGER_AGENT_HOST
• OTEL_EXPORTER_JAEGER_AGENT_PORT
• OTEL_EXPORTER_JAEGER_ENDPOINT
• OTEL_EXPORTER_JAEGER_USER
• OTEL_EXPORTER_JAEGER_PASSWORD
• OTEL_EXPORTER_ZIPKIN_ENDPOINT
• OTEL_TRACES_EXPORTER

[MrAlias]

Many of these environment variables require the ability to dynamically load a library if we do not want to have the SDK depend on everything. We could consider using https://pkg.go.dev/plugin to achieve this. Though, it should be fully evaluated as this looks like it will bypass many stability guarantees of the Go language and it is not compatible with all supported operating systems.

[MrAlias]

When implementing OTEL_LOG_LEVEL we will need to remain consistent with other language logging levels open-telemetry/opentelemetry-specification#2039

[MrAlias]

This issue is tracking too many things. I'm going to split it into component pieces so they can be addressed independently.

[MrAlias]

Closing and tracking this work with all the linked issues opened to replace this.