fixes #700 - Reorg Prometheus output for multiple caches+connectors

[StefanPenndorf]

• Reorganize output to match Prometheus requirements on help lines and
  statistics output
• Optimized sanitation by extracting patterns into constants
• Optimized/cleaned up controller methods

[evernat]

I have tried an alternative approach which is not bad I think.
It is attached here as diff and as plain source. (And I have included your change on patterns.)
The result is the same as yours (at least the same as StefanPenndorf@dbcedcf).
What approach would be better for your and why? (My code is shorter ;-)

javamelody.prometheus.diff.txt
PrometheusController.java.txt

[evernat]

This PR replaced by the alternative approach in 6f06e46 which is merged and ready for the next release (1.71).
Thanks anyway for testing the fixed result and for this PR approach.
There will be a new snapshot build available in about 24h here:
https://javamelody.ci.cloudbees.com/job/javamelody/net.bull.javamelody$javamelody-core/