Improve integration with Metrics plugin

[jamesrobson-secondmind]

This improves the metrics generated:

• Support the fallback causes logic, so only causes that will be reported for a build are counted
• Adds a count of builds that failed without matching any causes, like the old visualisations did

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[PayBas]

Nice 👍
Looking forward to this in the next release.

[jamesrobson-secondmind]

So I'm not sure what to do about testing this. There's currently no tests for the scan function or the fallback categories and this change is very simple, so I'm going to say no tests are needed unless someone objects to that.

[PayBas]

Well I'm currently using this PR in combination with my own #149 and while I am seeing:

# HELP jenkins_bfa_cause_no_matching_cause Generated from Dropwizard metric import (metric=jenkins_bfa.cause.no matching cause, type=com.codahale.metrics.Counter)
# TYPE jenkins_bfa_cause_no_matching_cause gauge
jenkins_bfa_cause_no_matching_cause 23.0

I'm not seeing the jenkins_bfa_category_### fallback categories. So either I integrated the 2 PRs incorrectly, or it's not working as intended.
Adding an automated test might help clear up this question ;)

[jamesrobson-secondmind]

This has some tests now, can it be merged?

[PayBas]

            if (!foundCauseList.isEmpty()) {
                incCounters(foundCauseList, PluginImpl.getInstance().isMetricSquashingEnabled());
            } else {
                incCounters(new ArrayList<>(Collections.singletonList(UNKNOWNCAUSE)),
                        PluginImpl.getInstance().isMetricSquashingEnabled());
            }

[PayBas]

Using this PR live for some time now. Looks good to me. But I'm not reviewer ;)