support expressions in assertions

[mathnogueira]

Problem

Today we only can execute assertions against static values. However, there are some use-cases for more complex assertions, such as asserting that an operation that is async runs in under 2 minutes. Or that your wallet balance is less than it was before the purchase.

Examples

myapp.user.balance < myapp.user.balance_before_purchase

span.endTime < tracetest.execution.startTime + 2m

Design

To make it fast, I think this should not allow using attributes from other spans. So, you can only reference attributes of the span that is being validated at that moment.