Add SubjectSet completeness metrics

[camallen]

closes #3450

TODO

• Add the flipper feature flag subject_set_completeness_from_read_replica to read the counts from the replica DB

This PR adds the ability to calculate the completeness of a subject set in the context of a workflow and record these completeness metrics on the SubjectSet resource in a json blob that is keyed by the workflow id.

As a subject set can belong to many workflows in a project we need to record the completeness metrics for the SubjectSet in the context of these workflows. Specifically we need to calculate the ratio of the number of retired subjects in a set divided by the the number of subjects in a set, formula completeness = total_retired_subjects_for_a_workflow_in_a_set / total_subjects_in_a_set.

Finally we clamp the completeness metric to a sensible range, 0.0 (0%) to 1.0 (100%) to ensure we display sensible numbers and avoid issues with incorrect numerators and denominators in the calculation (described in #2155)

These completeness metrics stores this per workflow metric a subject_set.completeness jsonb column in the database and uses the atomic jsonb update operators (available since our upgrade to pg v11) to avoid clobbering data already in the jsonb column and/or by concurrent updates.

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.

[yuenmichelle1]

Everything seems fine. I had a couple minor questions about syntax, nothing deal breaking.

One thing: how does this work with un-retirement? Do we need to call this SubjectSetCompletenessWorker in this case as well?

[camallen]

Everything seems fine. I had a couple minor questions about syntax, nothing deal breaking.

One thing: how does this work with un-retirement? Do we need to call this SubjectSetCompletenessWorker in this case as well?

Good catch about unretirement events - i'll update the PR to handle that event :)

[yuenmichelle1]

Looks good to me! 👍