Add more fine-grained support for selecting metric exemplars

[mhalbritter]

OpenTelemetry supports the OTEL_METRICS_EXEMPLAR_FILTER environment variable, which selects which spans can serve as exemplars.

Our org.springframework.boot.micrometer.tracing.autoconfigure.otlp.OtlpExemplarsAutoConfiguration.LazyTracingExemplarContextProvider uses the sampled flag from the span, which is essentially trace_based in OTel. We should also support always_on and always_off.

There's also code in org.springframework.boot.micrometer.tracing.autoconfigure.prometheus.PrometheusExemplarsAutoConfiguration.LazyTracingSpanContext. Prometheus supports only trace based sampling and always off.

[mhalbritter]

We'll rename the property to management.tracing.exemplars.include with values all, none, sampled-traces.

[mhalbritter]

We should also think about: if the property is none, we could just not register the ExemplarContextProvider and SpanContext beans, saving some memory and cycles.

[answndud]

I’d like to work on this.

My plan is to rename the property to management.tracing.exemplars.include, align the values with all, none, and sampled-traces, and update the OTLP and Prometheus auto-configurations and tests accordingly. I also plan to avoid registering the exemplar support beans when the value is none.

[answndud]

Opened PR #49594 with the property rename, conditional bean registration for none, and the related doc/test updates.

Tests run locally:

• ./gradlew :module:spring-boot-micrometer-tracing:test --tests org.springframework.boot.micrometer.tracing.autoconfigure.otlp.LazyTracingExemplarContextProviderTests --tests org.springframework.boot.micrometer.tracing.autoconfigure.prometheus.LazyTracingSpanContextTests :module:spring-boot-micrometer-tracing-brave:test --tests org.springframework.boot.micrometer.tracing.brave.autoconfigure.OtlpExemplarsAutoConfigurationTests --tests org.springframework.boot.micrometer.tracing.brave.autoconfigure.PrometheusExemplarsAutoConfigurationTests