Refactoring

[loelkes]

As I want to add some features and additional metrics I did some refactoring and cleanup.

• 9f2004b47757ad6f2290ca959f399fbe1185c076 Plugin hook form permissions system is now part of the plugin instance.
• 6a6be9cf7169c790c75eb943e29d9596aaeb18bc b83beb30ebbcf759baef2b12e31eb208d462f0b6 Move everything related to prometheus to a separate file
• 9f88130438f573d24be1c625946ac4825287c176 Only expose metrics about RaspberryPi core temperature if they are collected
• fddca08b889be5e5a2169c64f0fb9fb016bacf1d Rely on octoprint.util and SettingsPlugin Mixin for collecting metadata.

I would like to merge those before continuing with additional metrics.

[tg44]

LGTM 👍