Collective Impact initiatives are often tricky when it comes to evaluation because each partner justifiably has a wealth of metrics that (a) they already collect, and (b) want to start collecting once they see how easy it is to collect data on Makerble.
Rather than dampening down on this rare enthusiasm that people have to collect data, you can encourage it but manage it.
Allow people to
see the metrics that other partners have created (so they don't create duplicates)
Then
archive the ones you don't want to use
decide on a streamlined set of metrics that are used collectively by everyone as well as some peripheral metrics that are used by individual partners as they see fit.
That way everyone wins. You get aggregated data across the metrics that matter to you whilst at the same time empowering your partners to collect additional data that helps them advance their outcomes.