Make getValuesAtIntervalOp consume all chunk data in one pass.

This is mainly a small performance improvement, since we skip past the last
extracted time immediately if it was also the last sample in the chunk, instead
of trying to extract non-existent values before the chunk end again and again
and only gradually approaching the end of the chunk.

storage/metric/operation.go

@@ -161,12 +161,12 @@ func (g *getValuesAtIntervalOp) ExtractSamples(in model.Values) (out model.Value
 		lastExtractedTime := out[len(out)-1].Timestamp
 		in = in.TruncateBefore(lastExtractedTime.Add(1))
 		g.from = g.from.Add(g.interval)
-		if lastExtractedTime.Equal(lastChunkTime) {
-			break
-		}
 		for !g.from.After(lastExtractedTime) {
 			g.from = g.from.Add(g.interval)
 		}
+		if lastExtractedTime.Equal(lastChunkTime) {
+			break
+		}
 		if g.from.After(g.through) {
 			break
 		}

storage/metric/operation_test.go

@@ -1610,6 +1610,16 @@ func TestGetValuesAtIntervalOp(t *testing.T) {
 			t.Fatalf("%d. expected length %d, got %d: %v", i, len(scenario.out), len(actual), scenario.op)
 			t.Fatalf("%d. expected length %d, got %d", i, len(scenario.out), len(actual))
 		}
+
+		if len(scenario.in) < 1 {
+			continue
+		}
+		opTime := scenario.op.CurrentTime()
+		lastExtractedTime := scenario.out[len(scenario.out)-1].Timestamp
+		if opTime != nil && opTime.Before(lastExtractedTime) {
+			t.Fatalf("%d. expected op.CurrentTime() to be nil or after current chunk, %v, %v", i, scenario.op.CurrentTime(), scenario.out)
+		}
+
 		for j, out := range scenario.out {
 			if out != actual[j] {
 				t.Fatalf("%d. expected output %v, got %v", i, scenario.out, actual)