Add "free" memory to _astats

[smalenfant]

This Feature Request affects these Traffic Control components:

• Traffic Monitor
• Traffic Server astats plugin

Description

We have been running in a condition (which don't know root cause at this time) in which lots of Traffic Server on the CDN has been running into low memory conditions (down 50-60GB within hours) and available memory is exhausted.

Would be nice to have Traffic Monitor making the caches OFFLINE during those conditions to avoid failures

[smalenfant]

In addition, seems like proxy.process.traffic_server.memory.rss": 2787184640 is not representative of the current state of the ATS process memory being used.

USER        PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
ats        3443  147 97.5 412863996 384980056 ? SLl  Jan03 47199:42 /opt/trafficserver/bin/traffic_server...