Fixing metadata storing

[sovcik]

1. added dotenv as dev-dependency for easier testing
2. originally I wanted to fix dts too, so it won't be necessary to provide Promise with connected mongo-client, but left it as it is
3. fixed (I hope I understood your intention correctly) metadata storing - winston sends whole info object and metadata is everything besides label and message
4. fixed metadata key - you could provide a new name as an option, but was stored always as meta

[DABH]

I think this looks good to me. It's been a while since I looked at all the metadata stuff, but I do think this was the intent. Thank you for your contributions!