Convert metrics generation system tests to invoke per table

[patchwork01]

Background

Split from:

• Enable schedule rules by default in system tests #3722

Description

When we explicitly invoke metrics generation in system tests, we'd like to send a message per table of the current system test instead of invoking the trigger lambda.

This should allow us to run tests with an offline table, so that we can take control of the order things happen without needing to pause the whole instance.

We should be able to enable the metrics generation scheduled rule to make the tests more representative of a real system.

This should make it easier to parallelise tests in the future.

Analysis

See the parent issue for details of general code changes for schedule rules in system tests.