Make it more obvious what application id is used by an application

[snicoll]

With a vanilla setup, we start emitting metrics (and potentially traces) for unnamed_service in the unnamed_application. I wonder if we shouldn't make it more obvious so that the user has a chance to understand that:

• They should look for those IDs to find back metrics and traces for that app
• This can (and should be) configured.

I should be not intrusive, a single line INFO message should do.