Before digging into converting the backend to Prometheus, you may want to have a look at their actual persistence method. What worries me is that they are still storing one metric per file, and are copying data from one file to the next to perform checkpoints or purges (which is admittedly an improvement over whisper's method of thrashing back and forth across a single file).
They have the code to do in-place method of appending chunks, but in my admittedly brief foray through the code I could not actually find any use of it.
The graphite backend is pluggable. It isn't converting so much as adding support for. One of my favorite backends is Cyanite which ultimately stores the data in Cassandra.
They have the code to do in-place method of appending chunks, but in my admittedly brief foray through the code I could not actually find any use of it.