[Proposal] Use header `Server-Timing` instead of `xg-timing`

[masipcat]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing it's still a draft but it's implemented by all major browsers.

The main advantage using this header it's the graph in the browser:

We can keep xg-timing headers as well if you want

@bloodbare @vangheem sounds good?

[bloodbare]

Sounds great for me! You mean to render this headers allways or maintain the debug parameter?

[masipcat]

Maintain the debug parameter as it is now

[vangheem]

That looks great! I had no idea this was a feature!