Add telegraf version to User-Agent header (#4838)

Header is added in influxdb, influxdb_v2, and http outputs.

plugins/outputs/http/http.go

@@ -187,6 +187,7 @@ func (h *HTTP) write(reqBody []byte) error {
 		req.SetBasicAuth(h.Username, h.Password)
 	}
 
+	req.Header.Set("User-Agent", "Telegraf/"+internal.Version())
 	req.Header.Set("Content-Type", defaultContentType)
 	if h.ContentEncoding == "gzip" {
 		req.Header.Set("Content-Encoding", "gzip")