Logs with category "ApplicationGatewayFirewallLog" does not have proper category label in Loki

[Kapsztajn]

Describe the bug
This bug is similar to issue in OpenTelemetry Collector: open-telemetry/opentelemetry-collector-contrib#28806
Logs which are forwarder to Loki from Event Hub through Promtail does not have any category for ApplicationGatewayFirewallLog even though it is visible in Event Hub message.

{"records": [{ "timeStamp": "2024-09-18T00:45:09+00:00", "resourceId": "/SUBSCRIPTIONS/hidden/RESOURCEGROUPS/hidden/PROVIDERS/MICROSOFT.NETWORK/APPLICATIONGATEWAYS/hidden", "operationName": "ApplicationGatewayFirewall", "category": "ApplicationGatewayFirewallLog"

It is probably happening due to missing time field and relabel not properly working:
https://github.com/grafana/agent/pull/3412/files#diff-f7b899798d7f7b22d9a64584a767fa07952fa917226e312ad088de922ea06e71R37
Only timeStamp field is available for ApplicationGatewayFirewallLog.

Normal message from Application Gateway with proper category relabel looks like this:

{"records": [{ "timeStamp": "2024-09-18T00:45:20+00:00", "time": "2024-09-18T00:45:20+00:00", "resourceId": "/SUBSCRIPTIONS/hidden/RESOURCEGROUPS/hidden/PROVIDERS/MICROSOFT.NETWORK/APPLICATIONGATEWAYS/hidden", "listenerName": "hidden", "ruleName": "hidden", "backendPoolName": "hidden", "backendSettingName": "hidden", "operationName": "ApplicationGatewayAccess", "category": "ApplicationGatewayAccessLog"

To Reproduce
Steps to reproduce the behavior:

1. Run Promtail with Azure Event Hub
2. Generate some logs from Application Gateway WAF
3. Check if category is missing even though logs are visible in Loki/Grafana

Expected behavior
Proper category should be assigned/relabeled by Promtail in Loki for ApplicationGatewayFirewallLog even when only timeStamp filed is available in message.

Environment:

• Infrastructure: Azure Container Apps
• Deployment tool: Docker

Screenshots, Promtail config, or terminal output
Promtail config below:

server:
  http_listen_port: 9080
  grpc_listen_port: 0

clients:
  - url: https://hidden.grafana.net/loki/api/v1/push
    basic_auth: 
      username: hidden
      password: hidden

scrape_configs:
  - job_name: test
    azure_event_hubs:
      fully_qualified_namespace: hidden.servicebus.windows.net:9093
      connection_string: 'Endpoint=sb://hidden.servicebus.windows.net/;SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=hidden'
      event_hubs: 
        - hidden
      labels:
        job: azure_event_hub
    relabel_configs:
      - action: replace
        source_labels:
          - __azure_event_hubs_category
        target_label: category

[Kapsztajn]

@andriikushch tagging you here