This empty folder exists to harmonize “built-in” metrics work with the “metric extension” functionality of trace processor.

Essentially by adding a protos folder, we can pass src/trace_processor/metrics as an extension path to shell to override all built-in metrics. This means we can change the SQL files without needing to recompile trace processor.

In the future, we might also move the protos here but that would be a bigger change which would need coordination across repos ( e.g. Chrome because of the metrics autoroller for Chrome metrics).