Telemetry

[danschultzer]

Adds telemetry events to resolve #492

Events

• [x] Session ID creation
• [x] Session ID renewal
• [x] Session ID destruction
• [ ] Login and logout operations
• [ ] Privilige changes
• [ ] Timeout expiration
• [ ] Invalid session activities
• [ ] Critical business operations

Todo

• [ ] Conform docs to how Ecto and Phoenix lists events
• [ ] Check how :otp_app is handled, whether it's set as part of the event name or as a tag (I think the latter is better)