Add cost metrics retention period to Cloud Cost Management data retention docs (#37783)

Co-authored-by: jeff-morgan-dd <jeff.morgan@datadoghq.com>

Author: 2 people

content/en/data_security/data_retention_periods.md

@@ -52,6 +52,7 @@ attributes:
        - **Pipelines, stages, jobs, setups, commands**: 15 months
   - product: Cloud Cost Management
     data_type: |
+       - **Cost metrics**: 15 months
        - **Recommendations**: 90 days
   - product: Cloud Security
     data_type: |