prometheus metrics was using total disks instead
of online disk count, when disks were down, this
PR fixes this and also adds a new metric for
total_disk_count
also re-use storage disks for all `mc admin server info`
calls as well, implement a new LocalStorageInfo() API
call at ObjectLayer to lookup local disks storageInfo
also fixes bugs where there were double calls to StorageInfo()
We can use this metric to check if there are too many S3 clients in the
queue and could explain why some of those S3 clients are timing out.
```
minio_s3_requests_waiting_total{server="127.0.0.1:9000"} 9981
```
If max_requests is 10000 then there is a strong possibility that clients
are timing out because of the queue deadline.
