This is for GAE Flex Env, application written in Go.
Two parts to the question:
Where is it idiomatic to store application secrets such as passwords/keys to connect to other services? In Heroku for example they are env vars. But in GAE Flex you can't put them in app.yaml because that file has other config settings in it and it normally gets stored in git. Some ideas: Metadata service, Datastore, GCS bucket ... but see next question.
How do you deal with local application testing? To use Heroku as an example again, you can create a local .env file. Getting the setting at runtime is consistent, you just read env vars. How do you make this easy to use with GAE Flex? Any Go utilities/libraries out there that abstract local env vs. GAE Flex "production" env? Of course, there are usually (at least) 3 envs: local/dev, staging, production.
