"Failed to get disk metrics, permission denied" log message is unhelpful

[dekobon]

This message is output from src/core/metrics/sources/disk.go:43. What appears to be going on here is that we loop through each mount point and collect disk usage metrics based on each one. However, if the operation fails for a given mount point there we do not output for which mount point an error was encountered.

As such, I'd suggest a change where we replace line 43 with the following:

c.logger.Log(fmt.Sprintf("Failed to get disk metrics for mount point %s, %v", part.Mountpoint, err))

If everyone is amenable, I'll submit a PR for this.