Server-Timing

[Simbiat]

function for server-timing (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing https://www.smashingmagazine.com/2018/10/performance-server-timing/)

Probably needs to be a separate class and also be called from zEcho. Need support for:
memory usage (by current process), cpu usage (by current process), I/O, disk usage (by current process), script execution time, number of sql queries (from Database class, that may need an update), system uptime, number of functions/files called (?), support for custom entries.

Use hrtime() for time calculations? Add it to zEcho if a global variable for start time was set?

Need to also handle Timing-Allow-Origin to allow only specific origins (or * if empty)."