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Issue description

The default alert routing tree includes a match for alerts with severity=critical label.  If a custom route match is added after this one for any alert with that label, then the alert will not be routed to the receiver.  This can be tricky to troubleshoot and not obvious to see.

I'd recommend we add a small bit of content raising the use of `continue: true` to solve this sort of problem.

The Lab at the end of this chapter uses this, but doesn't mention it nor require the learner to pay it any attention.