Expose statistics

[markusressel]

Is your feature request related to a problem? Please describe.
Having the ability to get some graphical insight on the data of Grocy would be really cool.

Describe the solution you'd like
A prometheus reporter is already included in this bot. Since the data in Grocy is monitored for Notification purposes anyway, it would be nice to add gauges for:

• Products
  • number of products in inventory (per product name)
  • number of expired products
  • number of products below minimum
• Shopping List
  • number of items on the shopping list (per shopping list name)
• Chores
  • number of chore execution (per chore name)
  • number of overdue chores
• Tasks
  • number of open tasks

and possibly more.