[metricbeats] Enrich statistics about swap and memory

[unixsurfer]

Describe the enhancement:
Add the following metrics to the list of exposed memory/swap metrics
pgscank = pgscan_kswapd from /proc/vmstat, number of pages scanned by the kswapd daemon
pgscand = pgscan_direct from /proc/vmstat, number of pages scanned directly
pgfreed = pgfree from /proc/vmstat, number of pages freed by the system
pgsteal = pgsteal from /proc/vmstat, Number of pages placed on the free list by the system
We call GetVMStat to fetch a VMStatInfo struct but we only fetch few metrics, while GetVMStat returns the aforementioned struct which has all metrics from /proc/vmstat.

Moreover, it would be great if you can caluclate the vmeff metric that sar tool list:

  %vmeff
                     Calculated as pgsteal / pgscan, this is a metric of the efficiency  of  page
                     reclaim.  If  it  is near 100% then almost every page coming off the tail of
                     the inactive list is being reaped. If it gets too low (e.g. less  than  30%)
                     then  the virtual memory is having some difficulty.  This field is displayed
                     as zero if no pages have been scanned during the interval of time.

Describe a specific use case for the enhancement or feature:
These metrics will help us to have visibility in memory saturation cases

[unixsurfer]

Ideally, it would be great to expose everything that VMStatInfo struct contains.

[fearful-symmetry]

@unixsurfer these changes have been merged and should be in 7.6. I'm gonna close this, if there's any related changes can we open it in another issue?

[unixsurfer]

@fearful-symmetry Thanks a lot for work on this. It is very much appreciated.