Google Fonts: use dynamic browser user agent

[simison]

Impacted plugin

Jetpack

What

Switch static browser agent to dynamic one.

How

When requesting Google Fonts API at the backend, we pass a static browser user agent with the request:

jetpack/projects/packages/google-fonts-provider/src/class-google-fonts-provider.php

Line 91 in 063d4eb

$args = array( 'user-agent' => 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:73.0) Gecko/20100101 Firefox/73.0' );

Google Fonts API can return different response depending on the user agent (see docs).

As time goes by, our static agent likely won't get updated. Google API at some point start think they're responding to old browser and need to avoid sending some unsupported features.

Since the response is cached, it might make sense to include the browser name/version in the cache key. The whole user agent string in the key is likely too much.

The browser user agent is used at also at our own API implementation (pMz3w-g6E-p2/#comment-103419).

[simison]

These APIs are deprecated longer ago already.