Allow configuring envoy's application log format for better integration with observability tooling

[bartsmykla]

Description

It would be beneficial to allow configuring envoy application logging format (probably json would be enough), which would allow better integration with observability tooling (like Datadog)

---

ref. https://www.envoyproxy.io/docs/envoy/latest/configuration/observability/application_logging#printing-logs-in-json-format

application_log_config:
  log_format:
    json_format:
      Timestamp: "%Y-%m-%dT%T.%F"
      ThreadId: "%t"
      SourceLine: "%s:%#"
      Level: "%l"
      Message: "%j"
      FixedValue: "SomeFixedValue"

[github-actions]

This issue was inactive for 90 days. It will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant, please comment on it or attend the next triage meeting.

[github-actions]

This issue was inactive for 90 days. It will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant, please comment on it or attend the next triage meeting.

[jakubdyszkiewicz]

Triage: we found a way to configure this in Datadog itself (different processor)

[slonka]

it's in https://app.datadoghq.com/logs/pipelines -> name -> kuma-cp > steps 1 & 2

[github-actions]

Automatically closing the issue due to having one of the "closed state label".