SDK configuration from ENV Variable

[cijothomas]

Spec has documented a list of ENV variable names from which SDK should read config, if SDK chose to support ENV var based config.
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md

[cijothomas]

Given this is not a MUST, we can add this after-ga.

[pellared]

I would like to discuss the potential API and behavior changes that are expected as part of this issue.

I see two "types" of configuration that can be done via environmental variables.

The first one is about configuring a specific component like Zipkin Exporter configuration. The logic here can be simply encapsulated into existing packages like https://www.nuget.org/packages/OpenTelemetry.Exporter.Zipkin/. No API changes needed.

The second one is harder as it is about enabling components e.g. Exporter selection. I think this type of setting may even need some at least high-level design to make sure we have something that covers OTEL_PROPAGATORS. Taking the OTEL_TRACES_EXPORTER as an example I imagine we could add a new extension method to https://www.nuget.org/packages/OpenTelemetry.Exporter.Zipkin/ with something like (I am bad at naming):

public static TracerProviderBuilder RegisterZipkinExporterEnvVar(TracerProviderBuilder builder) {
    return builder.RegisterExporter("zipkin", () => builder.AddZipkinExporter());
}

Also, it may be worth creating a convenience NuGet package with a method like AddCommonEnvVarSelections which would "register" all env vars defined by the specs.