cmd/geth, metrics: separate process metric collection, add disk

metrics/disk_nop.go

@@ -0,0 +1,10 @@
+// +build !linux
+
+package metrics
+
+import "errors"
+
+// ReadDiskStats retrieves the disk IO stats belonging to the current process.
+func ReadDiskStats(stats *DiskStats) error {
+	return errors.New("Not implemented")
+}