Add DB_URL_FILE and DB_PASSWORD_FILE

[Munksgaard]

This allows passing sensitive DB passwords through files instead of environment variables. This makes collector work better with systemd credentials and NixOS flakes.

Fixes #540

[Munksgaard]

I am unsure if I should add similar functionality for PGA_API_KEY. I'm guessing this is considered sensitive information as well, but the logic around loading PGA_API_KEY seems a bit more complicated, so I would probably need some guidance.

[msakrejda]

Nice, thanks for the contribution. Thoughts:

• Should the _FILE versions silently override the explicitly specified strings like this?
• Should we be using TrimSpace? Technically your password could start or end with a space
• Should we support these from a file-based config, in addition to env-based config?
• There's currently no way to return an error from getDefaultConfig, but is panic the right error-handling strategy here?

[Munksgaard]

Hi @msakrejda!

These are all good questions, which I don't think I'm necessarily the right person to answer, but I'll give it a shot. To preface, I'll say that I don't have much experience with go, so forgive my ignorance.

1. I think that is how these things usually go, but I wouldn't mind adding a warning of some sort. How would you suggest I do that?
2. Great question. I would think that these values are usually trimmed, to remove any newlines at the end of the file, but you've got a great point that the password might contain spaces. I'm fine with removing the trimming if desired.
3. Sure, I don't see why not. How/where would I add that?
4. Great question. I'll leave that up to the maintainers to decide.

[msakrejda]

Thanks. I'll check in with my colleagues and we'll follow up.