Create ServiceMonitors for Prometheus exporters

[iblancasa]

When the Prometheus Exporter is used, the operator exposes the port associated to that exporter. But, in order to gather those metrics, the user needs to create a ServiceMonitor or PodMonitor resource.

It would be interesting if the OpenTelemetry Operator could create those resources for the user automatically. To ensure the feature is enabled only when the Prometheus CR is there, all the logic will be protected by the operator.observability.prometheus feature gate (implemented as part of #1768).

This will need the work done as part of #1953.