Show species by source for (x)releases on taxon pages

[mdoering]

Description

With taxon metrics we now have a map of source dataset keys and the number of accepted species they contribute.
It would be nice to show these on taxon details pages - maybe underneath the Taxonomic scrutiny section?

E.g.
https://api.dev.checklistbank.org/dataset/9910/taxon/NN/metrics
https://www.checklistbank.org/dataset/306220/taxon/NN

The metrics response also includes counts of accepted names by rank (taxaByRankCount) which we should use for the statistics that we currently get from ES. These cover all names incl synonyms which I think is less useful. And it would save us some slightly expensive call to facets.

Note that metrics only exist for taxa, not for synonyms!