The metrics catalog is a repository of community contributed notes on metrics you might want to be collecting grouped by protocol/service/application.
This is really more of a "best practices" repository.
The format of the repository is one directory per protocol/service/application. Each directory should contain at least a single README.md file that covers the information about it.
If need be, this can be converted to a Jekyll repository. That actually might make a lot of sense.
The owner of this repository is @obfuscurity. Please contact him with any questions or concerns.