cmd/scollector: Collect a new metric linux.disk.bytes

This will allow us to see bytes per second being written/read to a disk. We currently don't have this, we have sectors per second - but the size of the sector is unknown.
Swap the order of the merge here. opentsdb.TagSet{"stuff":"blah"}.Merge(ts). This is because Merge modifies the caller, ts in this case, which we don't want here.