Add count_scalar() function.

Change-Id: I63f09dd0479d0a6b016f5f857dd39dcbda56c7f9

rules/ast/functions.go

@@ -265,7 +265,18 @@ func scalarImpl(timestamp clientmodel.Timestamp, view *viewAdapter, args []Node)
 	return clientmodel.SampleValue(v[0].Value)
 }
 
+// === count_scalar(vector VectorNode) model.SampleValue ===
+func countScalarImpl(timestamp clientmodel.Timestamp, view *viewAdapter, args []Node) interface{} {
+	return clientmodel.SampleValue(len(args[0].(VectorNode).Eval(timestamp, view)))
+}
+
 var functions = map[string]*Function{
+	"count_scalar": {
+		name:       "count_scalar",
+		argTypes:   []ExprType{VECTOR},
+		returnType: SCALAR,
+		callFn:     countScalarImpl,
+	},
 	"delta": {
 		name:       "delta",
 		argTypes:   []ExprType{MATRIX, SCALAR},

rules/ast/printer.go

@@ -159,7 +159,7 @@ func EvalToString(node Node, timestamp clientmodel.Timestamp, format OutputForma
 		evalTimer.Stop()
 		switch format {
 		case TEXT:
-			return fmt.Sprintf("scalar: %v", scalar)
+			return fmt.Sprintf("scalar: %v @[%v]", scalar, timestamp)
 		case JSON:
 			return TypedValueToJSON(scalar, "scalar")
 		}

rules/rules_test.go

@@ -382,6 +382,18 @@ func TestExpressions(t *testing.T) {
 			output:         []string{`testcounter_reset_end => -90 @[%v]`},
 			fullRanges:     1,
 			intervalRanges: 0,
+		}, {
+			// count_scalar for a non-empty vector should return scalar element count.
+			expr:           `count_scalar(http_requests)`,
+			output:         []string{`scalar: 8 @[%v]`},
+			fullRanges:     0,
+			intervalRanges: 8,
+		}, {
+			// count_scalar for an empty vector should return scalar 0.
+			expr:           `count_scalar(nonexistent)`,
+			output:         []string{`scalar: 0 @[%v]`},
+			fullRanges:     0,
+			intervalRanges: 0,
 		}, {
 			// Empty expressions shouldn"t parse.
 			expr:       ``,