Let's start tackling T6041: Metrics for Maniphest with a very basic data point that most open projects will be interested in knowing:
How many active users do we have in Maniphest over time?
One way to calculate this could be: number of registered users performing any action in Maniphest tasks (creating, assigning, prioritizing, resolving, commenting, subscribing, awarding) on a given month. Then we could use Facts to show a graph of the evolution of this number on a daily basis.
How complex would be to build this?