Add ability to match on the length of an array.

Add ability to match on the length of an array.

By using the "magic" key value of __len, a match can be done
on the length of an array. For example:

  - filter:
      count: 1
      match:
        event_type: http
        http.response_headers.__len: 9

This does rely on us never using __len as an actual object
key, so is a bit of a hack.