Feature request: Hook that runs after (re)starting an app

[dreusel]

For many use cases, it would be awesome to have a hook which is executed after an app is (re)started, which could do all kinds of tasks.

My specific use-case is to retrieve relevant secrets from a vault system and injecting them the booting app. By doing this in a hook which runs within the PM2 God module process space, and applying some additional tricks, a setup can be constructed where PM2 plays the role of 'trusted intermediate': It will be the only entity in the system with access to the Vault system and deliver appropriate secrets to appropriate apps.

Other possible use-cases could be integration with a service-discovery system, service health monitoring, and many more