Alerting: Move templating to template package (#63347)

This commit moves templating from the state package to a sub-package
called template. This sub-package will be the logical package for
future ease-of-use improvements to templating custom annotations
and labels.

pkg/services/ngalert/state/template/template.go

@@ -0,0 +1,85 @@
+package template
+
+import (
+	"context"
+	"math"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/prometheus/common/model"
+	"github.com/prometheus/prometheus/pkg/timestamp"
+	"github.com/prometheus/prometheus/promql"
+	"github.com/prometheus/prometheus/template"
+
+	"github.com/grafana/grafana/pkg/services/ngalert/eval"
+)
+
+// Value contains the labels and value of a Reduce, Math or Threshold
+// expression for a series.
+type Value struct {
+	Labels map[string]string
+	Value  float64
+}
+
+func (v Value) String() string {
+	return strconv.FormatFloat(v.Value, 'f', -1, 64)
+}
+
+func NewValues(values map[string]eval.NumberValueCapture) map[string]Value {
+	m := make(map[string]Value)
+	for k, v := range values {
+		var f float64
+		if v.Value != nil {
+			f = *v.Value
+		} else {
+			f = math.NaN()
+		}
+		m[k] = Value{
+			Labels: v.Labels,
+			Value:  f,
+		}
+	}
+	return m
+}
+
+func Expand(
+	ctx context.Context,
+	name, tmpl string,
+	labels map[string]string,
+	res eval.Result,
+	externalURL *url.URL) (string, error) {
+	name = "__alert_" + name
+	tmpl = "{{- $labels := .Labels -}}{{- $values := .Values -}}{{- $value := .Value -}}" + tmpl
+	data := struct {
+		Labels map[string]string
+		Values map[string]Value
+		Value  string
+	}{
+		Labels: labels,
+		Values: NewValues(res.Values),
+		Value:  res.EvaluationString,
+	}
+
+	expander := template.NewTemplateExpander(
+		ctx, // This context is only used with the `query()` function - which we don't support yet.
+		tmpl,
+		name,
+		data,
+		model.Time(timestamp.FromTime(res.EvaluatedAt)),
+		func(context.Context, string, time.Time) (promql.Vector, error) {
+			return nil, nil
+		},
+		externalURL,
+		[]string{"missingkey=invalid"},
+	)
+	expander.Funcs(defaultFuncs)
+
+	result, err := expander.Expand()
+	// Replace missing key value to one that does not look like an HTML tag. This can cause problems downstream in some notifiers.
+	// For example, Telegram in HTML mode rejects requests with unsupported tags.
+	result = strings.ReplaceAll(result, "<no value>", "[no value]")
+
+	return result, err
+}