Add a Descriptor subinterface and implementation for Tor web server logs

The webstats-log-files are the only files available on CollecTor (in future) that are not yet covered by metrics-lib.

Should there be a 'LogDescriptor' interface and implementation in metrics-lib? Are there any reasons why not?

The name LogDescriptor is just a working name; better naming suggestions welcome. The interface should extend Descriptor and have additional methods for retrieving the measuring host and the served host, a method for retrieving the date of the log. What else?