Add callback for obtaining autoconfigured OpenTelemetry instance

[jack-berg]

Related to #5010.

If an autoconfigured SPI extension component like a SpanExporter, MetricExporter, or LogRecordExporter component wants to instrument itself with the instance of OpenTelemetry being autoconfigured, it current only option is to access GlobalOpenTelemetry.get() lazily. This doesn't work if the autoconfigured instance isn't set as global (i.e. AutoConfiguredOpenTelemetrySdkBuilder.setResultAsGlobal(false)).

A solution to this would be to add an interface like so:

public interface AfterAutoconfigureListener {
  void afterAutoconfigure(OpenTelemetry);
}

We could look for extension components which implement this (exporters, samplers, processors, etc), and invoke afterAutoconnfigure with the OpenTelemetry instance. We discussed this in the Java SIG and @trask hints at it again here.

[jack-berg]

Resolved in #5931.