os_release.go: Follow symlink to real os-release path.

Fixes: prometheus#2981

Author: jpds

collector/os_release.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
@@ -128,6 +129,11 @@ func parseOSRelease(r io.Reader) (*osRelease, error) {
 }
 
 func (c *osReleaseCollector) UpdateStruct(path string) error {
+	path, err := filepath.EvalSymlinks(path)
+	if err != nil {
+		return err
+	}
+
 	releaseFile, err := os.Open(path)
 	if err != nil {
 		return err