[sdk] Ability to add custom TextMapPropagators when using auto-configuration

[NthPortal]

I think it would be helpful (admittedly JVM only) to have an SPI for adding custom TextMapPropagators which can then be configured using environment variables.

Proposed SPI:

trait TextMapPropagatorProvider {
  def name: String
  def instance: TextMapPropagator[Context]
}

I've also opened open-telemetry/opentelemetry-java#7079 with a request for the equivalent in the official Java library.

I am aware that the sdk auto-configuration builder already supports custom TextMapPropagators, but purely environment-based configuration could be nice too

[iRevive]

Not ideal, but you can do the following:

OpenTelemetrySdk.autoConfigured[IO](
  _.addExportersConfigurer(OtlpExportersAutoConfigure[IO])
   .addTextMapPropagatorConfigurer(AutoConfigure.Named.const("myCustomPropagator", propagator))
)

Then you can dynamically configure it: export OTEL_PROPAGATORS=myCustomPropagator.
That doesn't improve usability, but it can be helpful if you need to configure the propagator dynamically.

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It will be interesting to explore the SPI topic in the SDK module. So we can finally get rid of the mandatory addExportersConfigurer.