Add resets() function to count counter resets.

resets() returns for every range vector element how many counter
resets there have been in the specified range.

promql/functions.go

@@ -505,6 +505,33 @@ func funcHistogramQuantile(ev *evaluator, args Expressions) Value {
 	return outVec
 }
 
+// === resets(matrix ExprMatrix) Vector ===
+func funcResets(ev *evaluator, args Expressions) Value {
+	in := ev.evalMatrix(args[0])
+	out := make(Vector, 0, len(in))
+
+	for _, samples := range in {
+		resets := 0
+		prev := clientmodel.SampleValue(samples.Values[0].Value)
+		for _, sample := range samples.Values[1:] {
+			current := sample.Value
+			if current < prev {
+				resets++
+			}
+			prev = current
+		}
+
+		rs := &Sample{
+			Metric:    samples.Metric,
+			Value:     clientmodel.SampleValue(resets),
+			Timestamp: ev.Timestamp,
+		}
+		rs.Metric.Delete(clientmodel.MetricNameLabel)
+		out = append(out, rs)
+	}
+	return out
+}
+
 var functions = map[string]*Function{
 	"abs": {
 		Name:       "abs",
@@ -621,6 +648,12 @@ var functions = map[string]*Function{
 		ReturnType: ExprVector,
 		Call:       funcRate,
 	},
+	"resets": {
+		Name:       "resets",
+		ArgTypes:   []ExprType{ExprMatrix},
+		ReturnType: ExprVector,
+		Call:       funcResets,
+	},
 	"round": {
 		Name:         "round",
 		ArgTypes:     []ExprType{ExprVector, ExprScalar},

promql/testdata/functions.test

@@ -0,0 +1,21 @@
+load 5m
+	http_requests{path="/foo"}	1 2 3 0 1 0 0 1 2 0
+	http_requests{path="/bar"}	1 2 3 4 5 1 2 3 4 5
+
+eval instant at 50m resets(http_requests[5m])
+	{path="/foo"} 0
+	{path="/bar"} 0
+
+eval instant at 50m resets(http_requests[20m])
+	{path="/foo"} 1
+	{path="/bar"} 0
+
+eval instant at 50m resets(http_requests[30m])
+	{path="/foo"} 2
+	{path="/bar"} 1
+
+eval instant at 50m resets(http_requests[50m])
+	{path="/foo"} 3
+	{path="/bar"} 1
+
+eval instant at 50m resets(nonexistent_metric[50m])