add seq label to storage_type

[stefanmonkey]

add seq label into storage_type in func collectStorageInfo to fix issue #2

after add , metrics output like

# HELP minio_storage_storage_type Minio backend storage type used
# TYPE minio_storage_storage_type gauge
minio_storage_storage_type{seq="0",type="Erasure"} 2
minio_storage_storage_type{seq="1",type="Erasure"} 2
minio_storage_storage_type{seq="2",type="Erasure"} 2
minio_storage_storage_type{seq="3",type="Erasure"} 2

[joe-pll]

Hi, after reading your PR I realised that the storageInfo are the same for all the servers. I sligthly modified the code. Could you please try to build from the branch 'server-data-info-fix' and tell me if you're still experiencing the problem? I started a 4 servers (16 exports) as you suggested to do and it works for me.

If it works for you too i'll merge it and build a new docker image. After that, can you please create another PR with the grafana dashboards?

[stefanmonkey]

@joe-pll I have create pr #5.

[rustup]

Great job