Allow UDFs to report stats about themselves

I have been working with a UDF locally and found that often I wanted to get more visibility into the behavior of my UDF. So I added some basic stat counters for various important events. This was nice but I had to expose those stats out of band from the normal socket connection and as a result it is difficult to map the stats back to a specific Kapacitor task.

Seems like two changes would be nice:

* Pass the Kapacitor task name down to the UDF during the startup connection steps. This way the UDF can record this context
* Allow a UDF to report arbitrary stats to Kapacitor and have them exposed on the UDF node.

Start by tracing the UDF startup connection steps and the UDF node's existing stats handling. Define how the Kapacitor task name is passed and how arbitrary UDF stats are exposed on the node, then verify both behaviors through the relevant UDF and task paths.

Written by the indexing model from the issue text.