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.