feat(processors.template): Add sprig function for templates (#16497)

Co-authored-by: Pieter Slabbert <pieter.slabbert@ilovezoona.com>

plugins/processors/template/README.md

@@ -7,6 +7,7 @@ routing option.
 
 The template has access to each metric's measurement name, tags, fields, and
 timestamp using the [interface in `/template_metric.go`](template_metric.go).
+[Sprig](http://masterminds.github.io/sprig/) helper functions are available.
 
 Read the full [Go Template Documentation][].
 

plugins/processors/template/template.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"text/template"
 
+	"github.com/Masterminds/sprig/v3"
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/processors"
 )
@@ -64,12 +65,12 @@ func (r *TemplateProcessor) Apply(in ...telegraf.Metric) []telegraf.Metric {
 func (r *TemplateProcessor) Init() error {
 	var err error
 
-	r.tmplTag, err = template.New("tag template").Parse(r.Tag)
+	r.tmplTag, err = template.New("tag template").Funcs(sprig.TxtFuncMap()).Parse(r.Tag)
 	if err != nil {
 		return fmt.Errorf("creating tag name template failed: %w", err)
 	}
 
-	r.tmplValue, err = template.New("value template").Parse(r.Template)
+	r.tmplValue, err = template.New("value template").Funcs(sprig.TxtFuncMap()).Parse(r.Template)
 	if err != nil {
 		return fmt.Errorf("creating value template failed: %w", err)
 	}

plugins/processors/template/template_test.go

@@ -305,3 +305,40 @@ func TestTracking(t *testing.T) {
 		return len(delivered) == 1
 	}, time.Second, 100*time.Millisecond, "%d delivered but 1 expected", len(delivered))
 }
+
+func TestSprig(t *testing.T) {
+	plugin := TemplateProcessor{
+		Tag:      `{{ .Tag "foo" | lower }}`,
+		Template: `{{ .Name | upper }}`,
+	}
+
+	err := plugin.Init()
+	require.NoError(t, err)
+
+	input := []telegraf.Metric{
+		testutil.MustMetric(
+			"cpu",
+			map[string]string{"foo": "MEASUREMENT"},
+			map[string]interface{}{
+				"time_idle": 42,
+			},
+			time.Unix(0, 0),
+		),
+	}
+
+	actual := plugin.Apply(input...)
+	expected := []telegraf.Metric{
+		testutil.MustMetric(
+			"cpu",
+			map[string]string{
+				"foo":         "MEASUREMENT",
+				"measurement": "CPU",
+			},
+			map[string]interface{}{
+				"time_idle": 42,
+			},
+			time.Unix(0, 0),
+		),
+	}
+	testutil.RequireMetricsEqual(t, expected, actual)
+}