fix: log full key def

[FluffyBucket]

Currently only the name from the spec would get logged if it was missing this makes it hard to easily find what exact value is missing for a given key.

Errors are given as: required key CLIENT_ID missing value, while the full key length is actually ECOM_CT_CLIENT_ID, where ECOM is the prefix and CT is the name of the config spec.

We do not need to use the alt name if it exists as the key field should always contain the key it uses to look up the env. Alt will only contain the name as specified in the envconfig struct tag.

[papkos]

👍 Agree, the "missing" error message should specify which env you need to set for the process for it to work, not just the name in the nested struct.

Tests need to be fixed, and I wonder if it would make sense to add 2 new:

1. One where there is no prefix, and no nesting
2. One still using the prefix, but nesting the missing ennvar in a labeled struct.

[hedlund]

Very nice! 👍

Remember to squash history before/when merging.