chore: add test case for SampleSnapshot.Sum

metrics/sample_test.go

@@ -103,20 +103,25 @@ func TestExpDecaySample(t *testing.T) {
 		}
 		snap := sample.Snapshot()
 		if have, want := int(snap.Count()), tc.updates; have != want {
-			t.Errorf("have %d want %d", have, want)
+			t.Errorf("unexpected count: have %d want %d", have, want)
 		}
 		if have, want := snap.Size(), min(tc.updates, tc.reservoirSize); have != want {
-			t.Errorf("have %d want %d", have, want)
+			t.Errorf("unexpected size: have %d want %d", have, want)
 		}
 		values := snap.(*sampleSnapshot).values
 		if have, want := len(values), min(tc.updates, tc.reservoirSize); have != want {
-			t.Errorf("have %d want %d", have, want)
+			t.Errorf("unexpected values length: have %d want %d", have, want)
 		}
+		sum := int64(0)
 		for _, v := range values {
+			sum += v
 			if v > int64(tc.updates) || v < 0 {
 				t.Errorf("out of range [0, %d]: %v", tc.updates, v)
 			}
 		}
+		if have, want := snap.Sum(), sum; have != want {
+			t.Errorf("unexpected sum: have %d want %d", have, want)
+		}
 	}
 }