properly strip path.rootfs from mountpoint labels (#1421)

Change-type: patch
Connects-to: #1418
Signed-off-by: dt-rush <nickp@balena.io>

collector/paths.go

@@ -15,6 +15,7 @@ package collector
 
 import (
 	"path/filepath"
+	"strings"
 
 	"github.com/prometheus/procfs"
 	kingpin "gopkg.in/alecthomas/kingpin.v2"
@@ -38,3 +39,10 @@ func sysFilePath(name string) string {
 func rootfsFilePath(name string) string {
 	return filepath.Join(*rootfsPath, name)
 }
+
+func rootfsStripPrefix(path string) string {
+	if *rootfsPath == "/" {
+		return path
+	}
+	return strings.TrimPrefix(path, *rootfsPath)
+}