[Metricbeat] Add new page statistics to system/memory metricset

[fearful-symmetry]

See #13744

This adds a new set of page scan and reclaim metrics to the system/memory metricset, as requested in the original issue. I'm a tad hesitant do dump the entire /proc/vmstat metrics in here, since this metricset is relatively clean and has a a fairly consistent format that I'd rather not dirty with a vmstat.* group.

These are fairly high-level metrics, so this adds a new page_stats group for them.

Metrics added:

• pgscan metrics, measuring pages scanned
• pgsteal metrics, measuring pages reclaimed
• pgfree metric, measuring pages freed by the system
• two efficiency metrics, measuring the efficiency of page reclaim.

With regards to efficiency, I was hesitant to to copy the vmeff metric from sar, as the pgscan and pgsteal metrics we get are much more detailed than what sar is collecting, so I instead copied the vmeff metric into groups, that measure the same thing (reclaim efficiency) at a more detailed level.

Test Plan

• Start metricbeat with the system/memory metricset enabled on a linux system
• Look for this field in the data:

           "page_stats": {
                "direct_efficiency": {
                    "pct": 0.9976
                },
                "kswapd_efficiency": {
                    "pct": 0.6213
                },
                "pgfree": {
                    "pages": 4382105954
                },
                "pgscan_direct": {
                    "pages": 485820
                },
                "pgscan_kswapd": {
                    "pages": 77390925
                },
                "pgsteal_direct": {
                    "pages": 484631
                },
                "pgsteal_kswapd": {
                    "pages": 48081976
                }
            },

[mtojek]

LGTM, but I believe this level knowledge deserves a short paragraph in README.

[unixsurfer]

@fearful-symmetry Thanks a lot for your work! It looks great, I only had on comment about the description for the efficiency field.

[fearful-symmetry]

Updated data.json and field descriptions.