promql: Add vector function.

Currently the only way to convert a scalar to a vector is to
use absent(), which isn't very clean. This adds a vector()
function that's the inverse of scalar() and lets your optionally
set labels.

Example usage would be
vector(time() % 86400) < 3600
to filter to only the first hour of the day.

promql/functions.go

@@ -650,6 +650,30 @@ func funcLabelReplace(ev *evaluator, args Expressions) model.Value {
 	return vector
 }
 
+// === vector(s scalar, vector model.ValVectora={}) Vector ===
+func funcVector(ev *evaluator, args Expressions) model.Value {
+	m := model.Metric{}
+	if len(args) >= 2 {
+		if vs, ok := args[1].(*VectorSelector); ok {
+			for _, matcher := range vs.LabelMatchers {
+				if matcher.Type == metric.Equal && matcher.Name != model.MetricNameLabel {
+					m[matcher.Name] = matcher.Value
+				}
+			}
+		}
+	}
+	return vector{
+		&sample{
+			Metric: metric.Metric{
+				Metric: m,
+				Copied: true,
+			},
+			Value:     model.SampleValue(ev.evalFloat(args[0])),
+			Timestamp: ev.Timestamp,
+		},
+	}
+}
+
 var functions = map[string]*Function{
 	"abs": {
 		Name:       "abs",
@@ -845,6 +869,13 @@ var functions = map[string]*Function{
 		ReturnType: model.ValVector,
 		Call:       funcTopk,
 	},
+	"vector": {
+		Name:         "vector",
+		ArgTypes:     []model.ValueType{model.ValScalar, model.ValVector},
+		ReturnType:   model.ValVector,
+		OptionalArgs: 1,
+		Call:         funcVector,
+	},
 }
 
 // getFunction returns a predefined Function object for the given name.

promql/testdata/functions.test

@@ -146,3 +146,18 @@ eval_fail instant at 0m label_replace(testmetric, "invalid-label-name", "", "src
 
 # label_replace fails when there would be duplicated identical output label sets.
 eval_fail instant at 0m label_replace(testmetric, "src", "", "", "")
+
+clear
+
+# Tests for vector.
+eval instant at 0m vector(1)
+  {} 1
+
+eval instant at 60m vector(time())
+  {} 3600
+
+eval instant at 0m vector(1, {a="b"})
+  {a="b"} 1
+
+eval instant at 0m vector(1, {a="b", c=~"d", e!="f", g!~"h", a="z"})
+  {a="z"} 1