Add meta stats fields

[tokee]

Running a web archive is often about managing scale. And about learning from experience when building the next iteration. Related to #205, which provides statistics aimed at quantitative analyses of content, we could use some index metrics:

• doc_term_count The total number of terms in all the fields in the document (copyFields in Solr might increase this a bit)
• doc_term_chars The total number of characters in all the terms in all the fields in the document (this will only be an approximation due to number fields)

This would help locating "large documents" and subsequently do qualified adjustments of field limits in the config file for the next full index.

Technically it would be simple to implement, as a post-analysis hook that iterates warc-indexer's Solr Document representation.