Allow reusing registered global OpenTelemetry

[quaff]

Spring Boot instrumentation will be included when applications are running with OpenTelemetry Java Agent.

[quaff]

Not sure reuse it by default or a boolean configuration property should be introduced.

[bclozel]

I don't think this is a sane goal. It is already complicated for Spring Boot users to figure out the available options.

We have tried to clarify things in #41227 (even if the reverse is not true) but if I understand correctly, this change will cause duplicate metrics and will disrupt OTel Java agent users.

[quaff]

this change will cause duplicate metrics and will disrupt OTel Java agent users.

I think it's necessary to introduce a flag to allow users reuse it.

EDIT: I've updated the PR to introduce configuration key management.opentelemetry.reuse-registered-global.

[wilkinsona]

Adding a flag doesn't really address Brian's point. We don't want users to be able to opt into having duplicate metrics. I'm going to close this one as the downsides outweigh any potential benefit. If we've misunderstood the implications of the change then please let us know and we can take another look.

[quaff]

Adding a flag doesn't really address Brian's point. We don't want users to be able to opt into having duplicate metrics. I'm going to close this one as the downsides outweigh any potential benefit. If we've misunderstood the implications of the change then please let us know and we can take another look.

FYI, I'm using OpenTelemetry Java Agent instrument traces only, and didn't configure management.otlp.tracing.endpoint to export spans at Spring Boot side, spans collected by Spring Boot will not be included without this change.
Of course we could reuse global OpenTelemetry in our own applications, but it's good if Spring Boot autoconfigure that, maybe I should add another condition to check OpenTelemetry Java Agent is not used for metrics (OTEL_METRICS_EXPORTER=none)?

[quaff]

Another option, we could reuse global registered OpenTelemetry only for Tracer if management.otlp.tracing.endpoint doesn't configured:

spring-boot/spring-boot-project/spring-boot-tracing/src/main/java/org/springframework/boot/tracing/autoconfigure/OpenTelemetryTracingAutoConfiguration.java

Lines 142 to 146 in 5db3cac

	@Bean
	@ConditionalOnMissingBean
	Tracer otelTracer(OpenTelemetry openTelemetry) {
	return openTelemetry.getTracer("org.springframework.boot", SpringBootVersion.getVersion());
	}

to

	@Bean
	@ConditionalOnMissingBean
	Tracer otelTracer(OpenTelemetry openTelemetry, Optional<OtlpTracingConnectionDetails> otlpTracingConnectionDetails) {
		OpenTelemetry otel = openTelemetry;
		if (GlobalOpenTelemetry.get() != OpenTelemetry.noop() && otlpTracingConnectionDetails.isEmpty()) {
			otel = GlobalOpenTelemetry.get();
		}
		return otel.getTracer("org.springframework.boot", SpringBootVersion.getVersion());
	}