What is the MessageLogging policy in Apigee and how is it used for audit and debugging?

<!-- MessageLogging to Google Cloud Logging -->
<MessageLogging name="ML.AuditLog">
  <CloudLogging>
    <LogName>projects/my-project/logs/apigee-api-access</LogName>
    <Message contentType="application/json">{
      "requestId": "{request.header.x-request-id}",
      "verb": "{request.verb}",
      "path": "{request.path}",
      "clientId": "{verifyapikey.VerifyAPIKey.client_id}",
      "statusCode": "{response.status.code}",
      "latency": "{target.elapsed.time}",
      "timestamp": "{system.timestamp}"
    }</Message>
    <Labels>
      <Label>
        <Key>environment</Key>
        <Value>{environment.name}</Value>
      </Label>
    </Labels>
  </CloudLogging>
</MessageLogging>

<!-- Key placement tip:
  MessageLogging is typically placed in the PostFlow Response
  AND in the DefaultFaultRule to capture both successful and
  error responses.
  The policy executes ASYNCHRONOUSLY by default --
  it does NOT block the response to the client. -->

<!-- Alternative: syslog (for on-prem logging systems) -->
<MessageLogging name="ML.Syslog">
  <Syslog>
    <Message>{request.verb} {request.path} - {response.status.code}</Message>
    <Host>logs.example.com</Host>
    <Port>514</Port>
    <Protocol>UDP</Protocol>
  </Syslog>
</MessageLogging>