Make interval property in PodMonitor configurable

[cmontemuino]

What would you like to be added:

To interval value in PodMonitor's spec.podMetricsEndpoints configurable from values.yaml.

Why is this needed:

Being able to fine tune the interval. Current value might be too aggressive or not, depending on the cluster, but also specific needs for monitoring.

This should be a very simple feature to implement, with no impact in current installations.

I'm open to contribute if required.

[TessaIO]

/assign