Fix metrics ism policy

[moesterheld]

Description

The gl-datanode-metrics-ism policy was created without an ism_template, so it was only attached to the backing indices of the gl-datanode-metrics data stream that existed at the time the policy was applied. Every backing index created by a later rollover stayed unmanaged, meaning the configured rollup and deletion never ran on it and indices and shards accumulated until they could trip cluster.max_shards_per_node. The policy now carries an ism_template for the metrics data stream, so OpenSearch attaches it to new backing indices automatically. Existing installations pick this up on the next Data Node restart, which rewrites the policy.

/jpd https://github.com/Graylog2/graylog-plugin-enterprise/pull/15363

Motivation and Context

fixes #27040

How Has This Been Tested?

added tests

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.