[exporter/prometheus] Add exemplar support for monotonic sum metrics in Prometheus exporter

[jlawrienyt]

Component(s)

exporter/prometheus

Is your feature request related to a problem? Please describe.

While working on the opentelemetry-demo project, adding exemplars to instrumentation, I discovered that the Prometheus exporter currently only adds exemplars to Histograms type metrics.

Describe the solution you'd like

To fully support exemplars, they should also been added, when available, to monotonic sum metrics.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• exporter/prometheus: @Aneurysm9

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[atoulme]

Looks like the work here is done, @jlawrienyt is it good to close?