Request a MongoDB lookup table in a dashboard

## What?
The idea is to be able to request a MongoDB lookup table in a bashboard, by using search query language or other appropriate way.

## Why?
Indeed, MongoDB lookup table can help to keep a sort of persistence that is unable to do by processing messages on the fly.

As explained here (https://community.graylog.org/t/graylog-capabilities/17729/10), my need is to follow and display on real-time in a dashboard the state of items (identified by an ID) where information comes form log messages analysis.
I succeeded to create a pipeline with rules that maintain a MongoDB lookup table that reflects the real state of my items.
But I'm unable to display this state because I'm unable to request a MongoDB lookup table in a dashboard.

## Your Environment
* Graylog Version: 3.3.5+6436f1b, codename Sloth Rocket