add MemAvailable and MemUsed metric 

What most admins want out of memory usage reporting is to know how much memory is used by running programs (vs. memory used by various kernel mechanisms) and how much is available for user programs to consume. Unfortunately, this is the one metric that is not provided by `/proc/meminfo`, which instead tells you just about memory used by the kernel. The MemAvailable and MemUsed metrics calculate the actual memory used or available for user programs.