Feature: Implement OpenMetrics.exemplars for the prometheus exporter

[danischroeter]

Storing some traceId's alongside prometheus metrics is a very powerful feature to analyze metrics and neatly integrate monitoring and tracing.
OpenMetrics,Prometheus and Grafana all have support for Exemplars to achieve just that.
This would very neatly also fit into Kamon.

see:
https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#exemplars
https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage
https://grafana.com/docs/grafana/latest/fundamentals/exemplars/