snmpd: support MemAvailable on Linux

[ibigbug]

This is my first time walking into this code base, so I don't know what I'm doing.

But the idea is to add support of reporting MemAvailable on Linux

-> % free -h
              total        used        free      shared  buff/cache   **available**
Mem:          3.8Gi       437Mi       2.5Gi       5.0Mi       936Mi       3.2Gi
Swap:         4.0Gi          0B       4.0Gi

please guide if I'm doing this correctly and/or anything that is not covered

[bvanassche]

Please use git rebase -i origin/master to merge the changes that address review comments into the first patch and share the combined patch with e.g. the following command: git push +support-linux-avail-mem

[csmall]

I'm not sure what the RFC says but putting my procps (free, top) hat on for a second the available figure is the most useful. There is a commit floating around somewhere from Linus about why this is a good value to use for memory pressure (e.g. how much more memory can I allocate before causing memory issues such as swapping).

[ibigbug]

done ping @bvanassche

[bvanassche]

A modified version of this patch has been applied. Please take a look at commit 5b8bf5d .

[ibigbug]

thanks @bvanassche

may I know when will this patch be released?

[bvanassche]

The date of the next release has not yet been decided as far as I know.

[ibigbug]

@bvanassche any chance to have a release recently? so this can be leveraged by librenms/librenms#12110