Track frequent/unsuccessful search queries in ckanext-stats

I'm interested in tracking what users search for, so that I can try to offer more datasets for these topics. Ideally I'd like to track both the queries and the number of results they returned so that I can identify topics which are requested often but for which no or only few datasets are available.

From my (limited) research it seems that Solr currently does not offer such a feature, so implementing this in CKAN (e.g. in ckanext-stats) seems like a good idea.

Start by reviewing ckanext-stats and CKAN's search flow, then determine where search queries and result counts can be observed. Done should include tracking query frequency and returned-result counts so frequently requested topics with few or no datasets can be identified.

Written by the indexing model from the issue text.