metadata: panic with additional information in rare panic case

[braydonk]

Description

This PR introduces a manual panic with extra information in a scenario where a very rare panic occurs in our test runs. I have been unable to reproduce the failure in manual attempts, so this should help us discover the same bug happening again and give us more visibility to potentially address in the future.

Related issue

b/477046139

How has this been tested?

I tested this by manually putting it into a unit test to mess with the message format. It will look like this:

panic: this failure case appears to occur at random in a way we can't reproduce. this panic exists to dump more information for the scenario where it occurs.
			metric name: example.googleapis.com/example.example1
			expected labels: [&{broker .*  []}, &{destination .*  []}]