metric to detect filesystems rules that don't match any local dataset (#653)

This PR adds a Prometheus counter called
`zrepl_zfs_list_unmatched_user_specified_dataset_count`.
Monitor for increases of the counter to detect filesystem filter rules that
have no effect because they don't match any local filesystem.

An example use case for this is the following story:
1. Someone sets up zrepl with `filesystems` filter for `zroot/pg14<`.
2. During the upgrade to Postgres 15, they rename the dataset to `zroot/pg15`,
   but forget to update the zrepl `filesystems` filter.
3. zrepl will not snapshot / replicate the `zroot/pg15<` datasets.

Since `filesystems` rules are always evaluated on the side that has the datasets,
we can smuggle this functionality into the `zfs` module's `ZFSList` function that
is used by all jobs with a `filesystems` filter.

Dashboard changes:
- histogram with increase in $__interval, one row per job
- table with increase in $__range
- explainer text box, so, people know what the previous two are about
We had to re-arrange some panels, hence the Git diff isn't great.

closes https://github.com/zrepl/zrepl/pull/653

Co-authored-by: Christian Schwarz <me@cschwarz.com>
Co-authored-by: Goran Mekić <meka@tilda.center>

daemon/filters/fsmapfilter.go

@@ -160,6 +160,14 @@ func (m DatasetMapFilter) Filter(p *zfs.DatasetPath) (pass bool, err error) {
 	return
 }
 
+func (m DatasetMapFilter) UserSpecifiedDatasets() (datasets zfs.UserSpecifiedDatasetsSet) {
+	datasets = make(zfs.UserSpecifiedDatasetsSet)
+	for i := range m.entries {
+		datasets[m.entries[i].path.ToString()] = true
+	}
+	return
+}
+
 // Construct a new filter-only DatasetMapFilter from a mapping
 // The new filter allows exactly those paths that were not forbidden by the mapping.
 func (m DatasetMapFilter) InvertedFilter() (inv *DatasetMapFilter, err error) {