[prometheusreceiver] Support exemplars from Prometheus

[dashpole]

Is your feature request related to a problem? Please describe.
Currently, the prometheus receiver drops prometheus exemplars:

opentelemetry-collector-contrib/receiver/prometheusreceiver/internal/otlp_transaction.go

Lines 103 to 105 in 5cd8dd9

	func (t *transactionPdata) AppendExemplar(ref storage.SeriesRef, l labels.Labels, e exemplar.Exemplar) (storage.SeriesRef, error) {
	return 0, nil
	}

Describe the solution you'd like
As described in the prometheus conversion specification, exemplars on prometheus metrics should be converted to exemplars in OpenTelemetry.

[dashpole]

Fixed by #12987

[bboreham]

This should be re-opened, as #12987 was reverted in #14000.

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

[jeromeinsf]

What's left to do after #14132 ?