[BUG] Existing indices are not assigned new ISM Policies

[fgendc]

Describe the bug
Adding an ISM policy with an index pattern doesn't assign existing indices to the policy.
According to the documentation here states in point 6 that's not the expected behavior.

To Reproduce
Steps to reproduce the behavior:

1. Create index 'default-1'
   curl -XPUT http://localhost:9200/default-1 -d '{ "settings": { "index": { "number_of_shards": 2, "number_of_replicas": 1 } }, "mappings": { "properties": { "age": { "type": "integer" } } }, "aliases": { "sample-alias1": {} } }' -H "Content-Type: application/json"
2. On Dashboards, create new ISM Policy with ISM template and index_pattern "default-*"
3. Index is not assigned the Policy and doesn't appear in the Managed Index section in Dashboards.
4. Create index 'default-2'
   curl -XPUT http://localhost:9200/default-2 -d '{ "settings": { "index": { "number_of_shards": 2, "number_of_replicas": 1 } }, "mappings": { "properties": { "age": { "type": "integer" } } }, "aliases": { "sample-alias1": {} } }' -H "Content-Type: application/json"
5. default-1 isn't assigned the policy, but default-2 has it.

Expected behavior
default-1 index matches the pattern defined in the ISM template section of the policy and should be assigned the policy after creating it.

Plugins

• opensearch-alerting
• opensearch-anomaly-detection
• opensearch-asynchronous-search
• opensearch-cross-cluster-replication
• opensearch-index-management
• opensearch-job-scheduler
• opensearch-knn
• opensearch-observability
• opensearch-performance-analyzer
• opensearch-reports-scheduler
• opensearch-security
• opensearch-sql
• repository-s3

Desktop (please complete the following information):

• OS: MacOS
• Version 12.2

Additional context
Installed on Kubernetes using Helm for both OpenSearch (1.2.4) and OpenSearch Dashboards (1.2.0), with Security Plugin disabled.

[bowenlan-amzn]

ISM template only works for indices created afterwards.

But you can hack it to manage earlier created index by changing the last_updated_time of ism_template to an earlier time epoch.

ism_template: {
  index_patterns: ...
  priority: ...
  last_updated_time: <earlier time epoch>
}