feat(outputs.mqtt): Add sprig for topic name generator for homie layout (#16491)

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

plugins/outputs/mqtt/README.md

@@ -201,6 +201,7 @@ while `homie_node_id` will provide a template for the `node-id` part of the
 topic. Both options can contain [Go templates][GoTemplates] similar to `topic`
 with `{{ .PluginName }}` referencing the metric name and `{{ .Tag "key"}}`
 referencing the tag with the name `key`.
+[Sprig](http://masterminds.github.io/sprig/) helper functions are available.
 
 For example writing the metrics
 

plugins/outputs/mqtt/homie.go

@@ -8,6 +8,8 @@ import (
 	"strings"
 	"text/template"
 
+	"github.com/Masterminds/sprig/v3"
+
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/internal"
 )
@@ -102,7 +104,7 @@ type HomieGenerator struct {
 }
 
 func NewHomieGenerator(tmpl string) (*HomieGenerator, error) {
-	tt, err := template.New("topic_name").Parse(tmpl)
+	tt, err := template.New("topic_name").Funcs(sprig.TxtFuncMap()).Parse(tmpl)
 	if err != nil {
 		return nil, err
 	}