Add user information to traces

[sissbruecker]

Providing user information would make it easier to debug issues reported by specific customers / users. This could either be the username, or the user id. We probably want to set this as early as possible, and as context, so that nested spans inherit the attribute.

This depends on what auth mechanism / framework the application uses. In a first step we could add support for Spring applications, which seem to have some generic mechanisms for retrieving user info.

This might require some more research regarding security / privacy implications.