Add offset to filebeat integration

[danielkasen]

It would be useful to add something like this to the gauge:

            self.gauge('filebeat.registry.offset', offset,
                       tags=['source:{0}'.format(source)])

Since the stats api only gives you the compressed output bytes and you might want the additional non-compressed bytes.

[yzhan289]

Checking in on this issue, pinging codeowner @wk8.