Integration with the Metrics plugin

[jamesrobson-secondmind]

This adds integration with the Metrics plugin to replace the graphs that were removed by version 2, this will allow users to integrate with there own metrics systems (e.g. Prometheus or Data Dog).
This uses a counter from the Metrics library which will reset when jenkins is restarted which was done for implementation simplicity.

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[malashi]

can this be merged now?

[jamesrobson-secondmind]

Yes, this is ready to merge now.

[PayBas]

I feel that there is a big omission here: the "Fallback categories"

[jamesrobson-secondmind]

Yes, that's a missing feature which I'll look at adding.

[jamesrobson-secondmind]

#148 fixes the fallback categories