Resolves #2797.

Main changes:

1. Consolidate some Prometheus formatting code so it can be uniformly maintained in one place and reused.
2. Add code to metrics/metrics:
   1. Save a Sample (value, label, timestamp instead of only a value) for each observation in a histogram data set, for the most recent counter update, and for the most recent simple timer update.
   2. Change the OpenMetrics/Prometheus formatting for histograms, simple timers, and counters to emit the sample (if it's present) as a comment following the rest of the output, consistent with exemplar formatting.
   3. Add the new ExemplarServiceManager which uses the ServiceLoader mechanism to load all implementations of the new ExemplarService, if any, ordered by the @Priority annotation on the implementing classes. Helidon Metrics uses the selected implementations to get label(s) to be stored with each sample. If the ServiceLoader finds no such service, the manager uses a simple default implementation that always returns an empty string.
3. Add a new component metrics/trace-examplar which supplies an ExemplarService implementation that supplies the current trace ID as the label.
4. Add new SE and MP doc pages under metrics for this feature.
5. Add a new SE example.

To trigger this behavior, users add a dependency on the new metrics/trace-examplar artifact and also add a dependency on the Helidon Zipkin or Jaeger integration.

Alternatively, a developer could provide a custom implementation of ExemplarService instead of the one in metrics/trace-exemplar.