Integrate with Elastic APM

Elastic has an official APM agent for PHP, but it's still under development and they advise against using it in production. Instead there are a few unofficial. Most popular being philkra/elastic-apm-php-agent.

There is also a WP plugin that integrates that. But since this looks like a very simple plugin, we could investigate integrating the agent to master-theme directly and avoid the extra dependency.

@TODO: Provide access to the current ELK instance