Expose and increase Shopping Cart functionality

[tammergalal]

For an upcoming course about custom span tags, custom method instrumentation, and generating metrics from spans....we need access to some interesting functionality to tell the story.

Currently the narrative we are looking to hit is that of a problem with the shopping cart. The shopping cart should sometimes fail, sometimes be successful through checkout, and also allow for custom tagging to expose cart totals.

The current cart implementation doesn't lend to demonstrating this functionality, nor being able to tag spans or methods associated.