Have trouble referencing values and log line in promtail template stage

Hi, I deployed promtail with helm chart to receive syslog from rsyslogd and send them to Loki. It works fine.
I want to manipulate the syslog format to enhance Loki performance, but fail to make the template stage work properly.
I have this sample log below with typical syslog labels like app_name, facility, severity and so on

I want to put some of those labels into the log line, and make the log line a json-like string, so I can reduce the usage of labels and enhance the performance, and still use |json to parse the log line.
Ideally, the log line should be like
{"app_name":"some app name","facility":"local7","proc_id":"9233","severity":"informational","message":"Event|6901|LOG_INFO|AMM|-|An action has been triggered by the NAE agent L2X-Watchdog"}
And some labels are kept like below

host: some_hostname

Under the extraScrapeConfigs in values.yaml, the relabel_configs part works as expected, but in the template stage, none of those key references returns any values. The log line becomes an empty string.

      - job_name: syslog
          listen_address:{{ .Values.extraPorts.syslog.containerPort }}
          use_incoming_timestamp: true
            cluster: some_cluster_name
            data_type: some_device-syslog
          - source_labels:
              - __syslog_message_hostname
            target_label: host
          - source_labels:
              - __syslog_message_severity
            target_label: severity
          - source_labels:
              - __syslog_message_facility
            target_label: facility
          - source_labels:
              - __syslog_message_app_name
            target_label: app_name
          - source_labels:
              - __syslog_message_proc_id
            target_label: proc_id
          - template:
              source: message
              template: '{{ .severity }} {{ .facility }} {{ .app_name }} {{ .proc_id }} {{ .Entry }}'
          - output:
              source: message

Below are what my empty log lines look like

Please tell me if I did something wrong. Thank you

The chart I use

cat Chart.yaml
apiVersion: v2
appVersion: 2.5.0
description: Promtail is an agent which ships the contents of local logs to a Loki
home: new user can only post two links
icon: new user can only post two links

  • email: lokiproject@googlegroups.com
    name: Loki Maintainers
  • name: unguiculus
    name: promtail
  • new user can only post two links
    type: application
    version: 6.0.0

I found the solution in Promtail - rewriting output (and using internal labels) - #2 by wrightsonm
The pack does exactly what I want