I think M3 is a necessary project for Uber. Open Source solutions like Graphite is really not designed for Uber's scale in this particular case. And yes,
What did M3 offer that existing projects at the time such as Prometheus and OpenTSDB did not?
The project page states "M3, a metrics platform, and M3DB, a distributed time series database, were developed at Uber out of necessity. After using what was available as open source and finding we were unable to use them at our scale due to issues with their reliability, cost and operationally intensive nature we built our own metrics platform piece by piece."[1]
The project page states "M3, a metrics platform, and M3DB, a distributed time series database, were developed at Uber out of necessity. After using what was available as open source and finding we were unable to use them at our scale due to issues with their reliability, cost and operationally intensive nature we built our own metrics platform piece by piece."[1]
[1] https://www.m3db.io/