Fix cluster level metrics

dropping cluster level metrics from all endpoints but /metrics

Should be a part of the /metrics/system endpoint:

```eventing_memory_quota {type="cluster"} 256
cbas_memory_quota {type="cluster"} 2154
balanced {type="cluster"} 0
rebalance_status {type="cluster"} 0
max_bucket_count {type="cluster"} 30
memory_quota {type="cluster"} 8731
percentage {type="cluster"} 30
percentage {type="cluster"} 30
rebalance_success {type="cluster"} 5
rebalance_start {type="cluster"} 5
fts_memory_quota {type="cluster"} 512
index_memory_quota {type="cluster"} 512
hddfree {type="cluster"} 606069487044
hddtotal {type="cluster"} 631322382336
hddquota_total {type="cluster"} 631322382336
hddused_by_data {type="cluster"} 5120177493
hddused {type="cluster"} 25252895292
ramused {type="cluster"} 46006591488
ramquota_used {type="cluster"} 24888999936
ramquota_used_per_node {type="cluster"} 8296333312
ramquota_total_per_node {type="cluster"} 9155117056
ramtotal {type="cluster"} 49270444032
ramquota_total {type="cluster"} 27465351168
ramused_by_data {type="cluster"} 5980584064```