Add slowlog metricset to redis

[a3dho3yn]

Collects slow queries from Redis

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[ruflin]

This seems to overlap with https://www.elastic.co/guide/en/beats/filebeat/6.3/filebeat-module-redis.html

[a3dho3yn]

I was looking for 'slowlog len' and then I found that it's helpful to have details about slowlog. So I added slowlog.len to the info metricset (#7695), and decided to add this new metricset.
But now I think this looks more relevant to logs (and filebeaat), and it's better to just add some missing fields to that filebeat module.