Custom instrumentation
Breadcrumbs
Breadcrumbs where initially implemented in our Front-end Javascript library with the idea that it would help debug an issue when you have a trail of breadcrumbs with (user) actions leading up to the error.
We then realized this could also be useful for back-end requests, where you can add a time-ordered list of actions leading up to the error. For example adding breadcrumbs through decision trees that are otherwise not instrumented.

Only the last 20 added breadcrumbs will be added to the current transaction.