Meaning of internal stats on "_kapacitor" internal stats

Hi!

I'm starting to use Kapacitor 1.3 with InfluxDB 1.0.0.
In order to get internal statistics I created a task with the following TICK script: `kapacitor_selfstats.tick`:
````
stream
|from()
.database('_kapacitor')
.retentionPolicy('autogen')
|influxDBOut()
````

Which is creating and sending internal stats to my InfluxDB correctly, but, there is some documentation about what each one mean?

````
edges
ingress
kapacitor
nodes
runtime
topics
````

Thanks,
Regards

Start with the kapacitor_selfstats.tick example in the issue and inspect how the _kapacitor database exposes the edges, ingress, kapacitor, nodes, runtime, and topics measurements. Document what each statistic represents and how users can interpret it, using the existing internal-statistics documentation structure if found. Done means each listed measurement has a clear explanation.

Written by the indexing model from the issue text.