[input.mongo] add fs size

[bdronneau]

Feature Request

Opening a feature request kicks off a discussion.

Proposal:

Hello,

I would like to add FsUsedSize and FsTotalSize to dbDataStats for mongodb input. These values are present in db.stats() (tested on mongo 4.0.x)

uu:ROLE> db.stats()
{
        "db" : "awesome",
        "collections" : 4,
        "views" : 0,
        "objects" : 55025651,
        "avgObjSize" : 2940.6856609684546,
        "dataSize" : 55881708978,
        "storageSize" : 99163236352,
        "numExtents" : 0,
        "indexes" : 22,
        "indexSize" : 9695956992,
        "fsUsedSize" : 314493155328,
        "fsTotalSize" : 622017689600,
        "ok" : 1,
        "operationTime" : Timestamp(1000000005, 6),
        "$clusterTime" : {
                "clusterTime" : Timestamp(1000000005, 6),
                "signature" : {
                        "hash" : BinData(0,"foo="),
                        "keyId" : NumberLong("bar")
                }
        }
}

Current behavior:

Values are not retrieved.

Desired behavior:

Have

• mongodb_db_stats_fs_total_size
• mongodb_db_stats_fs_used_size

Use case:

I would like to monitor disk space usage on my instance.