Expose Redis main thread cpu time, and scrape system time via INFO command. (#8132)

Exposes the main thread CPU info via info modules ( linux specific only )
(used_cpu_sys_main_thread and used_cpu_user_main_thread). This is important for:

- distinguish between main thread and io-threads cpu time total cpu time consumed ( check
  what is the first bottleneck on the used config )
- distinguish between main thread and modules threads total cpu time consumed

Apart from it, this commit also exposes the server_time_usec within the Server section so that we can
properly differentiate consecutive collection and calculate for example the CPU% and or / cpu time vs
wall time, etc...
