Add days_in_month function.

promql/functions.go

@@ -860,6 +860,17 @@ func funcVector(ev *evaluator, args Expressions) model.Value {
 	}
 }
 
+// === days_in_month(v vector) scalar ===
+func funcDaysInMonth(ev *evaluator, args Expressions) model.Value {
+	vector := ev.evalVector(args[0])
+	for _, el := range vector {
+		el.Metric.Del(model.MetricNameLabel)
+		t := time.Unix(int64(el.Value), 0)
+		el.Value = model.SampleValue(32 - time.Date(t.Year(), t.Month(), 32, 0, 0, 0, 0, time.UTC).Day())
+	}
+	return vector
+}
+
 // === day_of_month(v vector) scalar ===
 func funcDayOfMonth(ev *evaluator, args Expressions) model.Value {
 	vector := ev.evalVector(args[0])
@@ -966,6 +977,12 @@ var functions = map[string]*Function{
 		ReturnType: model.ValScalar,
 		Call:       funcCountScalar,
 	},
+	"days_in_month": {
+		Name:       "days_in_month",
+		ArgTypes:   []model.ValueType{model.ValVector},
+		ReturnType: model.ValVector,
+		Call:       funcDaysInMonth,
+	},
 	"day_of_month": {
 		Name:       "day_of_month",
 		ArgTypes:   []model.ValueType{model.ValVector},

promql/testdata/functions.test

@@ -398,3 +398,11 @@ eval instant at 0m month_of_year(vector(1456790399)) + day_of_month(vector(14567
 # 2016-03-01 00:00:00 March 1st in leap year.
 eval instant at 0m month_of_year(vector(1456790400)) + day_of_month(vector(1456790400)) / 100
   {} 3.01
+
+# Febuary 1st 2016 in leap year.
+eval instant at 0m days_in_month(vector(1454284800))
+  {} 29
+
+# Febuary 1st 2017 not in leap year.
+eval instant at 0m days_in_month(vector(1485907200))
+  {} 28