Log a notice when linking an existing user

Currently we log a notice when a user's successfully authenticated and when an account is created for an external user. AFAICT we don't log anything when a remote user is linked to an existing account and I thought that could prove useful.

I was also wondering if there was any reason the provider and authname are logged on user creation but not on authentication?

Thanks for your time!

Original report (for Open ID Connect)

Thanks for the super helpful module!

Problem/Motivation

Currently the module logs a number of error situations, but as far as I can tell we don't issue a notice on more mundane actions. I'm in a situation now where a client would like to track down all users who have logged in directly using Drupal's authentication, and it occurred to me one way to attempt that might be reconciling logs. I completely get that this is an edge case (: I just thought there might be an appetite for a little extra logging in general. My thoughts were to log a notice on the following events:

• creating a new user
• linking an existing user
• successful authentication

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes