Update/Fix PROM_APP_LABEL_REGEX

[Channyboy]

Update PROM_APP_LABEL_REGEX from "mp_app=\"[-/A-Za-z0-9]+\"" to something that appropriately reflects the possible tag values.

Either updates regex or use logic for parsing.
Perhaps using mp_app=".*(?<!\\\\)", may work.

Tag values are any character that is UTF-8.

This current regex was an artifact from Metrics 2.0 when the (mp)_app value was supposed to be (optionally) resolved from the context-root of the application.