[prometheus-exporter] Export Gauge exemplars

[fcollonval]

Is your feature request related to a problem?

The exemplars were added in #4094 and support for the histogram exemplars in the Prometheus exporter is added in #4178

Describe the solution you'd like

Export exemplars for Gauge in the prometheus-exporter

Describe alternatives you've considered

No response

Additional Context

Support for exemplars for Gauge metrics in Prometheus Python client has landed in prometheus/client_python#1063. But it has not been released yet (as of October 16th 2024). This is a requirement to fix this issue.

Would you like to implement a fix?

None