Change templateCaptureValue to support using template functions (#38766)

* Change templateCaptureValue to support using template functions

This commit changes templateCaptureValue to use float64 for the value
instead of *float64. This change means that annotations and labels can
use the float64 value with functions such as printf and avoid having to
check for nil. It also means that absent values are now printed as 0.

* Use math.NaN() instead of 0 for absent value

pkg/services/ngalert/state/cache.go

@@ -3,6 +3,7 @@ package state
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 	"sync"
@@ -110,16 +111,13 @@ func (c *cache) expandRuleLabelsAndAnnotations(alertRule *ngModels.AlertRule, la
 // and labels template.
 type templateCaptureValue struct {
 	Labels map[string]string
-	Value  *float64
+	Value  float64
 }
 
 // String implements the Stringer interface to print the value of each RefID
 // in the template via {{ $values.A }} rather than {{ $values.A.Value }}.
 func (v templateCaptureValue) String() string {
-	if v.Value != nil {
-		return strconv.FormatFloat(*v.Value, 'f', -1, 64)
-	}
-	return "null"
+	return strconv.FormatFloat(v.Value, 'f', -1, 64)
 }
 
 func expandTemplate(name, text string, labels map[string]string, alertInstance eval.Result) (result string, resultErr error) {
@@ -148,23 +146,31 @@ func expandTemplate(name, text string, labels map[string]string, alertInstance e
 		Value  string
 	}{
 		Labels: labels,
-		Values: func() map[string]templateCaptureValue {
-			m := make(map[string]templateCaptureValue)
-			for k, v := range alertInstance.Values {
-				m[k] = templateCaptureValue{
-					Labels: v.Labels,
-					Value:  v.Value,
-				}
-			}
-			return m
-		}(),
-		Value: alertInstance.EvaluationString,
+		Values: newTemplateCaptureValues(alertInstance.Values),
+		Value:  alertInstance.EvaluationString,
 	}); err != nil {
 		return "", fmt.Errorf("error executing template %v: %s", name, err.Error())
 	}
 	return buffer.String(), nil
 }
 
+func newTemplateCaptureValues(values map[string]eval.NumberValueCapture) map[string]templateCaptureValue {
+	m := make(map[string]templateCaptureValue)
+	for k, v := range values {
+		var f float64
+		if v.Value != nil {
+			f = *v.Value
+		} else {
+			f = math.NaN()
+		}
+		m[k] = templateCaptureValue{
+			Labels: v.Labels,
+			Value:  f,
+		}
+	}
+	return m
+}
+
 func (c *cache) set(entry *State) {
 	c.mtxStates.Lock()
 	defer c.mtxStates.Unlock()