fix(outputs.mqtt): Preserve leading slash in topic (#14582)

plugins/outputs/mqtt/mqtt_test.go

@@ -908,6 +908,11 @@ func TestGenerateTopicName(t *testing.T) {
 			pattern: "double//slashes//are//ignored",
 			want:    "double/slashes/are/ignored",
 		},
+		{
+			name:    "preserve leading forward slash",
+			pattern: "/this/is/a/topic",
+			want:    "/this/is/a/topic",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

plugins/outputs/mqtt/topic_name_generator.go

@@ -54,5 +54,8 @@ func (t *TopicNameGenerator) Generate(hostname string, m telegraf.Metric) (strin
 	if topic == "" {
 		return m.Name(), nil
 	}
+	if strings.HasPrefix(b.String(), "/") {
+		topic = "/" + topic
+	}
 	return topic, nil
 }