Maybe collapse this into the pattern we've used before which is UseDefaultOrValue? Wondering if that might read better. Alternatively, a good place is to set the default when the extensionservice is created and have the default be there.

Sorry I didn't see this comment when I merged :-(

Maybe something like:

func (s Setting) DurationOr(t time.Duration) time.Duration {
    if s.UseDefault() {
        return t
    }

    return s.val
}

One consideration is that the natural way to do this for a timeout.Setting is probably to specify a backup time.Duration, but here we have a backup timeout.Setting.

Maybe we could add something like:

package timeout

func FirstOf(settings ...Setting) Setting {
    for _, s := range settings {
        if !s.UseDefault() {
            return s
        }
    }

    return DefaultSetting()
}