[Feature] Add a flag opposite to userTimingWhitelist to specify what to block.

[Amerousful]

Feature/improvement

Original flag for idea: --userTimingWhitelist

It looks more convenient to block some special metrics from Google, Facebook, and so on. The weak point of whitelist that we can easily miss a new or unexpected metrics.

[soulgalore]

Thank you @Amerousful I'll add that. I've seen that I missed renaming that feature in sitespeed.io too, the correct name is userTimingAllowList.

[Amerousful]

You're welcome!
Yeap, this's a good point.

[Amerousful]

PR with update of documentation about userTimingBlockList - #4483