fix: prevent slice aliasing in LabelValues.With

metrics/internal/lv/labelvalues.go

@@ -10,5 +10,10 @@ func (lvs LabelValues) With(labelValues ...string) LabelValues {
 	if len(labelValues)%2 != 0 {
 		labelValues = append(labelValues, "unknown")
 	}
-	return append(lvs, labelValues...)
+	// Create a new slice to avoid aliasing the underlying array.
+	// Without this, sibling With calls (e.g., c1.With("a","1") and
+	// c1.With("b","2")) can overwrite each other's label values.
+	result := make(LabelValues, len(lvs), len(lvs)+len(labelValues))
+	copy(result, lvs)
+	return append(result, labelValues...)
 }

metrics/internal/lv/labelvalues_test.go

@@ -20,3 +20,17 @@ func TestWith(t *testing.T) {
 		t.Errorf("With does not appear to return the right thing: want %q, have %q", want, have)
 	}
 }
+
+func TestWithNoAliasing(t *testing.T) {
+	base := LabelValues{"a", "1", "b", "2"}
+	c1 := base.With("c", "3", "d", "4")
+	c2 := base.With("e", "5")
+
+	// c2 should not have overwritten c1's values
+	if c1[4] != "c" || c1[5] != "3" {
+		t.Errorf("c1 was corrupted by sibling With call: got %v", c1)
+	}
+	if c2[4] != "e" || c2[5] != "5" {
+		t.Errorf("c2 has wrong values: got %v", c2)
+	}
+}