How to use JSON stage with syslog?

Hello everyone,I’m using Grafana - Loki - Promtail to gather and view the Syslog messages and I’m trying to parse a specific part of these logs that are in the JSON format, but it’s not working. I’m not sure if this is even possible in the first place, to be honest so, any help is appreciated.

Config file:

server:
  http_listen_port: 9080 
  grpc_listen_port: 0

positions:
  filename: /tmp/positions.yaml

clients:
  - url: http://localhost:3100/loki/api/v1/push

scrape_configs:
- job_name: syslog
  
  syslog:
    listen_address: 0.0.0.0:514
    idle_timeout: 60s
    label_structured_data: no
    labels:
      job: "syslog"
  
  relabel_configs:
    - source_labels: ['__syslog_message_hostname']
      target_label: 'host'
    - source_labels: ['__syslog_connection_hostname']
      target_label: 'pod_hostname'
    - source_labels: ['__syslog_connection_ip_address']
      target_label: 'container_ip'
    - source_labels: ['__syslog_message_severity']
      target_label: 'severity'
    - source_labels: ['__syslog_message_facility']
      target_label: 'facility'
    - source_labels: ['__syslog_message_app_name']
      target_label: 'appname'
    - source_labels: ['__syslog_message_proc_id']
      target_label: 'procid'
    - source_labels: ['__syslog_message_msg_id']
      target_label: 'msgid'
 
  pipeline_stages:
    - match:
        selector: '{appname="deployer"}'
        stages:
          - json:
              expressions:
                user_id: user_id
                event: event
                instance_id: instance_id
                component: component 
          - labels:
              user_id:
              event:
              instance_id:
              component:

Sample log: