statMetricsActive is incorrect because we prune recursively.

[Dieterbe]

see #787 more specifically #787 (review)

copying here because I don't trust GH enough to retain the comments long after the fact.

in our implementation, a path can be both a leaf and have children.
e.g. foo.bar can have data/metricdef (making it a leaf), while foo.bar.baz also exists (making foo.bar a branch)
In graphite this doesn't work, but some of our customers wanted this behavior, and it seemed fairly easy to support, so that's why we do.

the m.delete call is recursive: it deletes the path, and any sub-paths (I think i actually just discovered a related bug here), so what we should do is track the actual amount of paths that got deleted by the delete call, or I think better: the pruning process shouldn't recursively delete, but rather only delete leaves (and clean up stale branch nodes that no longer have children nor defs)