Cache size in mb metrics is not reporting correct numbers

[abhishekdas99]

the cluster has 1 node and has 74 gb local space. percentage of space used for cache is set to 20%.

In the node if we run df -h, its reporting correctly

[ec2-user@ip-10-0-0-122 ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       45G   32G   13G  73% /
devtmpfs        7.4G  2.9M  7.4G   1% /dev
tmpfs           7.4G     0  7.4G   0% /dev/shm
/dev/xvdab       37G  6.3G   31G  18% /media/ephemeral1
/dev/xvdaa       37G  6.3G   31G  18% /media/ephemeral0

But the numbers reported as metrics is not right .It reporting 53gb used

Element | Value
-- | --
metrics_rubix_bookkeeper_cache_size_mb_gauge_Value{instance="master",job="bookkeeper"} | 0
metrics_rubix_bookkeeper_cache_size_mb_gauge_Value{instance="node0001",job="bookkeeper"} | 53288

We need to push right numbers as part of cache_size_in_mb metrics

[abhishekdas99]