Ability to add custom `TextMapPropagator`s when using auto-configuration

[NthPortal]

Problem

Currently, there is no way (short of ugly hacks)^† to add custom TextMapPropagators when using AutoConfiguredOpenTelemetrySdkBuilder. However, TextMapPropagator is the natural and idiomatic way to propagate certain request information, such as theoretical X-Example-Foo and X-Example-Bar headers. This leaves users with the choice of either using programmatic configuration, losing the advantages of automatic configuration (such as cross-platform consistency and ease of change deployment), or of manually propagating their organisation-specific request information without using a TextMapPropagator.

Solution 1: Add a method

It would be simple to add a method to AutoConfiguredOpenTelemetrySdkBuilder (#addAdditionalPropagators, for example) to support adding additional custom propagators.

Pros:

• Easy to implement
• Easy for applications to use

Cons:

• Not extensible to current or future cross-platform automatic or environment-based configuration
• Changing application behaviour requires recompiling the application (or manually adding your own environment-based configuration)

Solution 2: SPI

An SPI such as the following could be used to allow applications or libraries to supply TextMapPropagators which could then be enabled using the OTEL_PROPAGATORS environment variable.

// CC0
public interface TextMapPropagatorProvider {
  String getName();
  TextMapPropagator getInstance()
}

If the name is provided by users, it should be strongly recommended ("SHOULD", RFC 2119) that the name be domain-namespaced (e.g. com.example.foobar). Alternatively, the propagator's name could be the fully-qualified class name of the value returned by getInstance(); however, this makes configuration largely platform-dependent.

Pros:

• Potentially extensible to other platforms in the future
• Only requires a jar on the classpath and a change to an environment variable to use—potentially no code needed by applications

Cons:

• More work to implement in SDK
• Slightly more work for the implementers of custom TextMapPropagators

Alternatives

The current alternatives I'm looking at are using the pure-Scala backend in otel4s or using the awful hack listed in the footnote.

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Somewhat related is the following discussion: open-telemetry/opentelemetry-java-instrumentation#9489

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^† ugly hack

show

public class Customizer implements BiFunction<TextMapPropagator, ConfigProperties, TextMapPropagator> {
    private AtomicBoolean added = new AtomicBoolean(false);
    private Iterable<TextMapPropagator> propagators;

    public Customizer(Iterable<TextMapPropagator> propagators) {
        this.propagators = propagators;
    }

    @Override
    public TextMapPropagator apply(TextMapPropagator textMapPropagator, ConfigProperties configProperties) {
        if (!added.getAndSet(true)) {
            // please excuse my less-than-pretty code; I primarily write Scala
            List<TextMapPropagator> toAdd = new ArrayList<>();
            toAdd.add(textMapPropagator);
            for (TextMapPropagator propagator: propagators) {
                toAdd.add(propagator);
            }
            return TextMapPropagator.composite(toAdd);
        } else {
            return textMapPropagator;
        }
    }
}

[jack-berg]

Solution 1: Add a method

See https://github.com/open-telemetry/opentelemetry-java/blob/main/sdk-extensions/autoconfigure/src/main/java/io/opentelemetry/sdk/autoconfigure/AutoConfiguredOpenTelemetrySdkBuilder.java#L154

Solution 2: SPI

See https://github.com/open-telemetry/opentelemetry-java/blob/main/sdk-extensions/autoconfigure-spi/src/main/java/io/opentelemetry/sdk/autoconfigure/spi/ConfigurablePropagatorProvider.java

[NthPortal]

@jack-berg I don't believe the method you linked reasonably allows one to add propagators (although you can using the ugly hack I mentioned). however, that SPI should work, thanks! it would be helpful to mention that SPI at the end of this section, though I see now that it is mentioned at the end of the page.

thanks!