Added functionality to expose prometheus metrics

[scne59]

I have added a "/metrics" page in order to be able to collect metrics with prometheus.

Output looks like this with instance name derived from config "Title"

brewpi_beer_temp{instance="testmachine"} 0
brewpi_beer_set{instance="testmachine"} 20.00
brewpi_fridge_temp{instance="testmachine"} 0
brewpi_fridge_set{instance="testmachine"} 20.00
brewpi_room_temp{instance="testmachine"} 0