Add support for ROLE_SESSION_KEY

[josegonzalez]

Upstream metadataproxy has support for ROLE_SESSION_KEY, which allows you to override the session name. It would be awesome to be able to change it to read the session name from a container label instead, allowing for better introspection as to what created the role.

Is this something you would take a pull request for?

[jippi]

Hey! Sure, a PR for that would definitely be accepted :)