We too use ceilometer and gnocchi and it works great. The old way with mongo simply didn't work.

Our environment is around 10k running instances and gnocchi currently has data for 1,667,072 resources with 6,879,910 metrics.

We are happy with gnocchi and so far haven't needed to change anything but would help contribute if there were things we needed.

Would be great if we could just maintain gnocchi as opposed to making yet another thing.

Sam 

Glad to hear that you are going well with Gnocchi, and I also hope
Gnocchi could be maintained as expected.

Just to be clear, adding MongoDB support back doesn't mean you have to
install and maintain that once you upgrade Ceilometer in the future, we
will guarantee the existing deployments won't be affected, it's optional