[Feature Request] Porting InfluxQL Integral Aggregator to Kapacitor InfluxQLNode

Working through the Kapacitor documentation and looking through the source code, it appears that Integral doesn't have an InfluxQL implementation in Kapacitor. It is the only InfluxQL Aggregation function that hasn't been ported to Kapacitor.

Start with the Kapacitor documentation and the existing InfluxQL aggregation implementations in the source, then trace how the InfluxQLNode exposes aggregation functions. Compare the existing ports with Integral and confirm the expected behavior from InfluxQL. Done means Integral is available as an InfluxQL aggregation in Kapacitor.

Written by the indexing model from the issue text.