Performance metrics are getting enabled by Puppeteer automatically when creating an instance of the Page class. That is the only reason why the Performance.enable method is listed for Gutenberg tests. They never retrieve the metrics. As such we don't have to implement this domain for the alpha release.

Also the basics for this method have been implemented for the remote protocol, which simply sets the internal flag to enabled. Maybe that is all what we should do here, and simply close this bug? Or shall we wait until the necessary event listeners have been added?

Andreas, what is your take on that?

That’s a good find. It means we don’t have to do any further
implementation work on this domain for the puppeteer-alpha.

We should leave this bug open until we support the API, but make
sure it’s not blocking anything for the MVP or Gutenberg.

As discussed with Andreas we have the implementation done for Performance.enable() and this CDP endpoint can be called. Handling (registering, unregistering) of events will be handled on their own meta bugs.

The implementation was done for Firefox 67 on bug 1533682.