collections: compute number of records using "count" API instead of "search"

[alejandromumo]

          Given that we only need to get the total n. of results, and that this task runs quite often for ALL collections, I would suggest using the `count` APIs of OpenSearch rather than a classic search, as it is more performant.

You might want to keep the search_records method in the service, and add a second one count_records.

Originally posted by @ntarocco in #1853 (comment)