tools/cephfs_mirror: Add datasync_queue_wait_duration metric

tools/cephfs_mirror: Add datasync_queue_wait_duration metric

Add the metric which measures the time spent by the snapshot
in the data queue waiting for the datasync threads.

Sample output:
When still 'waiting' in queue
{
    "/d1": {
        "state": "syncing",
        "current_syncing_snap": {
            "id": 18,
            "name": "d1_snap5",
            "sync-mode": "delta",
            "avg_read_throughput_bytes": "0.00 B/s",
            "avg_write_throughput_bytes": "0.00 B/s",
            "crawl": {
                "state": "in-progress",
                "duration": "13s"
            },
            "datasync_queue_wait": {
                "state": "waiting",
                "duration": "12s"
            },
            "bytes": {
                "sync_bytes": "0.00 B",
                "total_bytes": "110.99 MiB",
                "sync_percent": "0.00%"
            },
            "files": {
                "sync_files": 0,
                "total_files": 3719,
                "sync_percent": "0.00%"
            },
            "eta": "calculating..."
        },
        "last_synced_snap": {
            "id": 15,
            "name": "d1_snap4"
        },
        "snaps_synced": 0,
        "snaps_deleted": 0,
        "snaps_renamed": 0
    },
}
---------------
After 'complete'
{
    "/d1": {
        "state": "syncing",
        "current_syncing_snap": {
            "id": 18,
            "name": "d1_snap5",
            "sync-mode": "delta",
            "avg_read_throughput_bytes": "11.66 MiB/s",
            "avg_write_throughput_bytes": "34.55 MiB/s",
            "crawl": {
                "state": "completed",
                "duration": "17s"
            },
            "datasync_queue_wait": {
                "state": "completed",
                "duration": "19s"
            },
            "bytes": {
                "sync_bytes": "149.94 MiB",
                "total_bytes": "149.94 MiB",
                "sync_percent": "100.00%"
            },
            "files": {
                "sync_files": 5000,
                "total_files": 5000,
                "sync_percent": "100.00%"
            },
            "eta": "0s"
        },
        "last_synced_snap": {
            "id": 15,
            "name": "d1_snap4"
        },
        "snaps_synced": 0,
        "snaps_deleted": 0,
        "snaps_renamed": 0
    }
}
-----
Also stored in last_sync_snap section
{
    "/d1": {
        "state": "idle",
        "last_synced_snap": {
            "id": 18,
            "name": "d1_snap5",
            "crawl_duration": "17s",
            "datasync_queue_wait_duration": "19s",
            "sync_duration": "44s",
            "sync_time_stamp": "8172.009480s",
            "sync_bytes": "149.94 MiB",
            "sync_files": 5000
        },
        "snaps_synced": 1,
        "snaps_deleted": 0,
        "snaps_renamed": 0
    }
}

Fixes: https://tracker.ceph.com/issues/73453
Signed-off-by: Kotresh HR <khiremat@redhat.com>