Check for cancellation on every step of a range evaluation

Signed-off-by: Bryan Boreham <bryan@weave.works>

promql/engine.go

@@ -791,6 +791,9 @@ func (ev *evaluator) rangeEval(f func([]Value, *EvalNodeHelper) Vector, exprs ..
 	seriess := make(map[uint64]Series, biggestLen) // Output series by series hash.
 	tempNumSamples := ev.currentSamples
 	for ts := ev.startTimestamp; ts <= ev.endTimestamp; ts += ev.interval {
+		if err := contextDone(ev.ctx, "expression evaluation"); err != nil {
+			ev.error(err)
+		}
 		// Reset number of samples in memory after each timestamp.
 		ev.currentSamples = tempNumSamples
 		// Gather input vectors for this timestamp.