User defined metric and KPI

[bonndan]

As a user I want to define my own metrics and apply KPIs on them in order to address custom needs. For instance, I want to measure all pull requests in gitlab that have been opened by renovate.

Acceptance criteria:

• a user can use own metrics based on available data (i.e. basic data or gathered by link resolvers)
• if feasible, the metric should be configurable. If not, a java plugin structure is required Add plugin structure #14