promql: handle NaN in changes() correctly

promql/functions.go

@@ -788,7 +788,7 @@ func funcChanges(ev *evaluator, args Expressions) model.Value {
 		prev := model.SampleValue(samples.Values[0].Value)
 		for _, sample := range samples.Values[1:] {
 			current := sample.Value
-			if current != prev {
+			if current != prev && !(math.IsNaN(float64(current)) && math.IsNaN(float64(prev))) {
 				changes++
 			}
 			prev = current

promql/testdata/functions.test

@@ -50,6 +50,15 @@ eval instant at 50m changes(http_requests[50m])
 
 eval instant at 50m changes(nonexistent_metric[50m])
 
+clear
+
+load 5m
+  x{a="b"} NaN NaN NaN
+  x{a="c"} 0 NaN 0
+
+eval instant at 15m changes(x[15m])
+  {a="b"} 0
+  {a="c"} 2
 
 clear