Document how to define and use new SLIs
The goal of this issue is to deliver documentation that an engineer can follow to set a custom SLI on an endpoint in the codebase.
This should be documentation on the API we settle on for #1221 (closed) and document how to add it to the metrics catalog. Making sure it matches the SLO for the service.
The SLI definition in the runbooks will depend on the work in #1228 (closed)
The documentation will probably live in the development documentation
Edited by Rachel Nienaber