Monitor filesystem usage in metricbeat

[henrikno]

It would be nice if the metricbeat elasticsearch module could report file system usage as reported by Elasticsearch. It'd allow to alert when going low on disk and tracking usage over time.

      "fs" : {
        "timestamp" : 1491553781646,
        "total" : {
          "total_in_bytes" : 67371577344,
          "free_in_bytes" : 51743817728,
          "available_in_bytes" : 48291119104
        },

It is reported by ES stats here:

beats/metricbeat/module/elasticsearch/node_stats/_meta/test/node_stats.522.json

Lines 355 to 357 in c2bd3ef

	"total_in_bytes" : 67371577344,
	"free_in_bytes" : 51743817728,
	"available_in_bytes" : 48291119104,

The store.size.bytes is not accurate enough when indices are closed or not loaded.

[ruflin]

@henrikno Please have a look at #5230