[TC/CBBS] Move metric generation into tools-common

In MB-56327 we wrote some code to autogenerate metrics_metadata.json & boilerplate metrics code for the backup service. This was quite specific to the backup service however (e.g. the package name was hard coded, the JSON and code was generated from definitions in code).

For continuous backup we will also want metrics so we should move this generation into tools-common and make it more general.