An optional 'volume' parameter can be supplied to specify which volume's free space to display when more than one volume exists. The value of the parameter must be in form of `volume_N`, e.g. to display free space for volume2, `volume_2` should be set as 'volume' value. If omitted, first returned volume's free space will be shown (not guaranteed to be volume1).
To access these system metrics you need to connect to the DiskStation (`DSM`) with an account that is a member of the default `Administrators` group. That is because these metrics are requested from the API's `SYNO.Core.System` part that is only available to admin users. In order to keep the security impact as small as possible we can set the account in DSM up to limit the user's permissions inside the Synology system. In DSM 7.x, for instance, follow these steps: