release notes (#1845) (#1847)

(cherry picked from commit d8f88b0)

Signed-off-by: Eric <menwe@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

release-notes/opensearch-observability.release-notes-2.16.0.0.md

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+## Version 2.16.0.0 Release Notes
+
+Compatible with OpenSearch 2.16.0
+
+### Features
+
+- Register system index descriptors through SystemIndexPlugin.getSystemIndexDescriptors ([#1837](https://github.com/opensearch-project/observability/pull/1837))
+
+### Maintenance
+
+- Increment version to 2.16.0-SNAPSHOT ([#1834](https://github.com/opensearch-project/observability/pull/1834))