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Famous databases with custom metrics: Oracle DB , Postgres , Oracle Coherence , MS SQL Server , IBM DB2

Examples of custom metric usages:

• Some metrics may not be provided yet and user registers own.
• Metrics for load balancing.
• Various metrics of business processes or related to user application.
• Administration metrics.
• Security-related metrics.

Implementation proposal

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1. Custom metrics are experimental.
2. Custom Metrics base on the New Metric System.
3. Allow to register essential ready-to-use metrics like LongMetric which can be exported by a providing metric exporter.
4. User can add own metrics into different registries.
5. Custom metrics are separated from the internals by registry name prefix "custom." (lower cased). 
6. Names of custom metrics and their registries can't contain any spaces.
7. Management of custom metrics might require a permission.
8. If we expose only few simple metrics, we should document examples of effecient metrics implementations like LongAdder.

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