As designed in bug 1635238 and atop the MLA defined in bug 1646165 (and probably on top of the C++ API of bug 1642646), we're gonna need a JS API.
- Generating the specific metrics APIs for all current metric types
- probably via a custom outputter on glean_parser like we did for the first pass at the Rust API in bug 1635260
- Supporting multiple processes
- Likely get this "for free" because the underlying Rust API supports IPC (or will later)
- A regression suite for the custom glean_parser outputter
- unit tests (xpcshell?) for using generated APIs to record data and test values
- In-tree API docs with examples would be nice, but this can be taken as a follow-up