Add intermediate metrics for API and ext.echo.ui loading response times
Add intermediate metrics for API and ext.echo.ui loading response times

And add a FIXME for our existing metric possibly being wrong if the API
request finishes before the ext.echo.ui module is loaded.

Change-Id: I918187dd276193b7602f60527b423ca06cb7e2d0

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo