The creation of the Theme Previews page showed that the way I was doing
metrics for database model objects being created wasn't very good.
Whenever someone loaded the Theme Previews page, the "total topics"
metric would increase by 2, "total comments" by 4, and "exemplary
labels" by 1, because the page is creating that many fake objects and
the metrics were being sent in each class's __init__ method.
This changes to take advantage of SQLAlchemy's object lifecycle event
for "pending to persistent", which only triggers when an object is
actually persisted to the database. When this event happens, the
object's _update_creation_metric method is called, and all metric
updates have been moved into that method now.
Deimos