feat: add Used SSD stats

[dcloud9]

Our use case is to create Grafana alerts on:

1. Should set XX% of ssd capacity as critical threshold, which can be already be achieved by

weka_fs{environment="<env>", stat="ssd_percent_used"} > XX

2. If metadata is using more than YY% of ssd capacity we should be alerted, which can be achieved if used_ssd_metadata and used_ssd_data are exposed. For example only

(weka_fs{environment="prd", stat="used_ssd_metadata"} / weka_fs{environment="prd", stat="available_ssd"}) * 100 > YY