Intent to ship: @font-face metrics override descriptors

[Jonathan Kew]

As of today, I intend to turn the @font-face metrics override
descriptors[1] on by default for all platforms.

These have been developed behind the
layout.css.font-metrics-overrides.enabled preference (currently enabled
on Nightly builds).

Status in other browsers:
Already shipping in release versions of Blink-based browsers.

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1704494

This feature was previously discussed in this "Intent to prototype" thread:
https://groups.google.com/g/mozilla.dev.platform/c/zk_-KRtKJr0

JK