Feature request: Add Examplars to Span metrics

[fstab]

Include Exemplars in Span metrics to allow navigation from Span metrics to example traces.

For context, see https://opentelemetry.io/docs/specs/otel/metrics/sdk/#exemplar.

There are two scenarios:

• OBI is used for both Span metrics and tracing. In that case it should be easy to add Exemplars.
• OBI is used for Span metrics, and a runtime-specific instrumentation library (like the OpenTelemetry Java agent) is used for tracing. It would be cool if Beyla could include the trace IDs coming from the runtime-specific instrumentation as Exemplars. However, this is not easy.