Document best practice about feature gates and deprecated features.

[bogdandrutu]

I think would be helpful to document the expected workflow for how to update the collector and how to update config (e.g. for the expandvar deprecation). I propose that we tell users:

1. First do the upgrade of the collector with the gate disable to preserve the old behavior;
2. Once the upgrade is done and stable, update the config to use the new expandvar syntax and revert the feature gate;

[mx-psi]

Sounds like a good idea to me