Allow setting an empty set of labels for the allSlice() of a labeled metric in postConfigure

[devromik]

Currently, the following code would fail:

metricRegistry.timer(withName("service", "processing", "time"));

metricRegistry.postConfigure(metricWithName("service.processing.time"),
    modifying().timer(withTimer().labels(SERVICE)
        .allSlice().labels());

It is still possible to achieve this by defining a new slice with no labels, but this can be inconvenient. If we allow no labels in the total slice, we should forbid a total instance with the same name as allSlice to avoid name conflicts.