How to integrate Monolog with New Relic in Drupal 9

composer.json

{
  "require": {
    "drupal/monolog": "^2.2",
    "newrelic/monolog-enricher": "^2.0"
  }
}

monolog.services.yml

parameters:
  monolog.channel_handlers:
    default:
      handlers:
        - name: "new_relic_buffered"
          processors:
            - "drupal_message_placeholder"
            - "new_relic_processor"
        - name: "new_relic_error"

services:
  monolog.processor.new_relic_processor:
    class: NewRelic\Monolog\Enricher\Processor

  # Log API New Relic Processor
  # Has build in buffering for New Relic requests.
  monolog.handler.new_relic_log:
    class: NewRelic\Monolog\Enricher\Handler
    arguments: ["200", false]

  # Log API New Relic Handler (buffered)
  # Buffered handler waits for all messages before batch sending.
  monolog.handler.new_relic_buffered:
    class: Monolog\Handler\BufferHandler
    arguments: ["@monolog.handler.new_relic_log"]

  # Error Event New Relic Handler
  # Uses newrelic_notice_error to send logs to the Triage Errors inbox.
  monolog.handler.new_relic_error:
    class: Monolog\Handler\NewRelicHandler
    arguments: ["300", false]

  # Rotating file
  # Store logs in Pantheon's default log directory.
  monolog.handler.rotating_file:
    class: Monolog\Handler\RotatingFileHandler
    arguments: ["/logs/php/application.log", "15", "200", false]

settings.php

<?php

// Pantheon specific settings.
if (!empty($_ENV['PANTHEON_ENVIRONMENT'])) {

  // Load monolog config.
  $monolog_services = __DIR__ . '/monolog.services.yml';
  if (file_exists($monolog_services)) {
    $settings['container_yamls'][] = $monolog_services;
  }
  
}