Fix docs: explain available metrics for Hubble VS Legacy control plane

[SRodi]

Issue

The doc https://retina.sh/docs/Metrics/modes/ does not explain / differentiate between retina control planes.

We should clarify adv_ metrics are not available for Hubble control plane

This relates to #1055

[SRodi]

It seems this is an issue with the docs as the current implementation supports Advanced/Pod-Level metrics (i.e. metrics with the prefix adv_) only on Legacy control plane. However, I do not find this mentioned anywhere in the docs https://retina.sh/docs/Metrics/modes/