Adds extra docs to flehs out ParseLabels implementation (#7562)

Adding some extra comments for archaeological purposes

pkg/logql/syntax/parser.go

@@ -202,7 +202,18 @@ func ParseLabels(lbs string) (labels.Labels, error) {
 	if err != nil {
 		return nil, err
 	}
+	// Sort labels to ensure functionally equivalent
+	// inputs map to the same output
 	sort.Sort(ls)
 
+	// Use the label builder to trim empty label values.
+	// Empty label values are equivalent to absent labels
+	// in Prometheus, but they unfortunately alter the
+	// Hash values created. This can cause problems in Loki
+	// if we can't rely on a set of labels to have a deterministic
+	// hash value.
+	// Therefore we must normalize early in the write path.
+	// See https://github.com/grafana/loki/pull/7355
+	// for more information
 	return labels.NewBuilder(ls).Labels(nil), nil
 }