[CBBS] Auto generate metric_metadata.json

What is the problem?
Currently our metric_metadata.json - used to describe the prometheus metrics we expose - is handwritten. This was done in the interests of time. It leaves us open to forgetting to update it if more metrics are added however.

What is the solution?
Autogenerate this file. The metrics definitions in statistician/statistician.go already have most of the information we need. It should be possible to write a Go script that imports them and generates the file.