Add metrics on tasks not accepted due to busy slot

[eolivelli]

We should have a metric that tells how many tasks are not accepted, per user, due to "busy slot".

This way we can know if periodic activities driven by slots are skipping cycles due to system overload.

[eolivelli]

cc @aluccaroni

[aluccaroni]

To be meaningful I think this metrics should be per user AND per slot (if the problem is only one very slow slot we need a way to discern it from the rest)

[eolivelli]

Sure