[SQL] PostgrSQL server : CPU and Memory

I am a developer and I want to monitor metrics related to CPU and Memory of PostgreSQL server only using SQL queries.

Do we have any system tables which can give us the data? I have found this one 
https://aaronparecki.com/2015/02/19/8/monitoring-cpu-memory-usage-from-postgres 

but dont know whether it is exactly the same what I need. 

I am able to fetch the same using scripts (batch/shell) but getting data from SQL will be my main task.   
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