Use the prefix id for keying into metrics rules

Fixes a bug where we were looking for incorrect keys for metrics rules.
Previously, we were using full language names in the keys. Now, we use
the short language names in the metric ids. This is done through a
simplification of the code.

Also, this change does two smaller things:

1. Prints out the baseline count to the logs
2. Adds the `assertNever` function to ensure we never miss a case in
   a switch statement. This function is borrowed from vscode-codeql.

Author: aeisenberg