Show size of data under lookup table data adapter

It would be great if I could see how many entries are currently in the data (file) of a data adapter so I can properly tune my cache.

For example, I have a CSV file of about 2500 lines that will only rarely return a result. This means that I want to have all those lines in the cache to avoid constant disk lookups. Currently, I have to go to disk and do a `wc -l` do find this out, but it would be great if the data adapter could figure this out, too. This will obviously only work for pre-fetch type data adapters.

Alternatively, as a though, a cache that could always fetch all entries (with a bound) would be really cool, too. For now, I'll just set it to the number of lines on disk, plus some buffer to be safe in case the file grows in the future.