[BUG] When upgrading a 2.0-2.4 domain to 2.5+ domain, there is a monitor creation/update error

[lezzago]

What is the bug?
The source_to_query_index_mapping attribute was introduced in 2.5. If the cluster has been upgraded with existing document level monitors, those monitors would execute and would end up updating the alerting config index mapping in the source_to_query_index_mapping field, but would not include the enabled attribute. When the user tries to update or create a monitor, the plugin will try to update the alerting config index mapping, but would fail due to the source_to_query_index_mapping field already existing and not having the enabled field.

How can one reproduce the bug?

1. Create a OpenSearch cluster in 2.0-2.4 and create document level monitors that are running.
2. Upgrade cluster to 2.5+
3. Attempt to create/update a monitor.