MINOR: counters: Review conditions to increment counters from analysers

MINOR: counters: Review conditions to increment counters from analysers

Now, for these counters, the following rules are followed to know if it must be
incremented or not:

  * if it exists for a frontend, the counter is incremented
  * if stats must be collected for the session's listener, if the counter exists
    for this listener, it is incremented
  * if the backend is already assigned, if the counter exists for this backend,
    it is incremented
  * if a server is attached to the stream, if the counter exists for this
    server, it is incremented

It is not hardcoded rules. Some counters are still handled in a different
way. But many counters are incremented this way now.