lustre2: don't fail plugin if can't read brw_stats

Before this commit, the plugin entirely fails when run as anyone other
than root, because brw_stats files are symlinks to /sys/kernel/debug/.

Which was causing errors like this:
```
2024-03-22T02:40:40Z E! [inputs.lustre2] Error in plugin: failed to read file /proc/fs/lustre/osd-ldiskfs/MGS/brw_stats: open /proc/fs/lustre/osd-ldiskfs/MGS/brw_stats: permission denied
```

After this change, the plugin continues to run and gather whatever
metrics it can, even if brw_stats files are unreadable.

plugins/inputs/lustre2/lustre2.go

@@ -30,8 +30,9 @@ type tags struct {
 // Lustre proc files can change between versions, so we want to future-proof
 // by letting people choose what to look at.
 type Lustre2 struct {
-	OstProcfiles []string `toml:"ost_procfiles"`
-	MdsProcfiles []string `toml:"mds_procfiles"`
+	OstProcfiles []string        `toml:"ost_procfiles"`
+	MdsProcfiles []string        `toml:"mds_procfiles"`
+	Log          telegraf.Logger `toml:"-"`
 
 	// used by the testsuite to generate mock sysfs and procfs files
 	rootdir string
@@ -525,7 +526,12 @@ func (l *Lustre2) getLustreProcBrwStats(fileglob string, wantedFields []*mapping
 
 		wholeFile, err := os.ReadFile(file)
 		if err != nil {
-			return fmt.Errorf("failed to read file %s: %w", file, err)
+			if strings.Contains(err.Error(), "permission denied") {
+				l.Log.Debugf("failed to read file %s: %w", file, err)
+				continue
+			} else {
+				return fmt.Errorf("failed to read file %s: %w", file, err)
+			}
 		}
 		lines := strings.Split(string(wholeFile), "\n")