Context:
Follow up from INC-674 post mortem.

We need a more reliable way to identify Optimize mode during migration. To achieve this, we want to save the Optimize mode to ES as part of the metadata. If no mode is persisted yet, Optimize should write it to ES. If a mode is already written to ES, Optimize validates its running in the specified mode and throws an error if not.

We also need to add a validation in the migration for this introduction in case of rolling upgrades so we can rely on the field not being null in following migrations, see incident doc note: "When customer starts on a version before this new field was added to the metadata and upgrading several versions at once, the field will still be null in the following migrations during the rolling upgrade → we still cannot rely on the mode being present in the metadata"
This can either be done by stopping and logging an error during the migration if the field is null, requesting customer to start Optimize once, or by requesting the customer input the mode manually.

AT:

• field for Optimize mode is added to metadata
• if field is null during first migration after addition of this field, logic is implemented that results in the field being updated before migration is run