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Implementation proposal

General approach

• Custom metrics might be experimental.
• Custom Metrics are based on the New Metric System.
• Allow to register essential ready-to-use metrics like LongMetric which can be exported by a existing metric exporter.
• User can register own metrics into different registries.
• Custom metrics are separated from the internals by registry name prefix "custom.".
• Naming strategy of metrics and their registries are the same as of the provided metrics.
• Management of custom metrics might require a permission.
• We should document examples of effecient metrics implementations like LongAdder.

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/**
 * Manages custom metrics. 
 *
 * Metrics are grouped into registries. Every metric has full name which is conjunction of registry name and metric name. 
 * Registry name is part of the full name until last '.'. Metric name is the last part of the full name. 
 * 
 * Full names and registry names are always prefixed with "custom.". For example, if provided full name is "a.b.c.mname", 
 * it is automatically converted to "custom.a.b.c.mname". If full name is "custom.a.b.c.mname", it is used as is. 
 * Where "custom.a.b.c" is a registry name and "mname" is a metric name. 
 */

@IgniteExperimental
public interface IgniteCustomMetrics {
    /**
     * Registers metric. Adds new metric registry if absent. 
     *
     * @return Previous metric if it already exists. Otherwise, {@code metric}. 
     */

    <T extends Metric> T metric(String fullName, T metric). */;
    /** Removes metric

    void removeMetric(String fullName). */;
    /** @return Certain metric registry

    ReadOnlyMetricRegistry registry(String registryName) and all its metrics. */;
    /** Removes certain registry

    void removeRegistry(String registryName);


    // Essential metrics. Int, Long and Double should be enough.
 Return   /**
     * Registers a {@link LongMetric}. Adds new metric registry if absent.
     *
     * @return Previous long value setter if it already exists. A new one otherwise.
     */
   current or new metric.
 
 LongConsumer longMetric(String fullName, @Nullable String description)     */;
    /**
     * Registers a {@link DoubleMetric}. Adds new metric registry if absent.
     *
     * @return Previous double value setter if it already exists. A new one otherwise.


    DoubleConsumer doubleMetric(String fullName, @Nullable String description)/**;
    
     * Registers a {@link IntMetric}
. Adds new metric registry if absent.
     *
     * @return Previous int value setter if it already exists. A new one otherwise.
     */
    IntConsumer booleanMetric(String fullName, @Nullable String description);



    // Might be useful the gauges.
 Return   /**
     * Registers a {@link LongMetric} metric with value supplier {@code valueSupplier}. Addstrue if new metric registryis ifregistered absent.
     *
     * @return {@code True}of false if metric was added. {@code False} if mericis already exists.  
     */

    boolean longMetric(String fullName, LongSupplier valueSupplier, @Nullable String description)/**;
    
     * Registers a {@link DoubleMetric} metric
 with value supplier {@code valueSupplier}. Adds new metric registry if absent.
     *
     * @return {@code True} if metric was added. {@code False} if meric already exists.  
     */
    boolean doubleMetric(String fullName, DoubleSupplier valueSupplier, @Nullable String description);
    /**
     * Registers a {@link IntMetric} metric with value supplier {@code valueSupplier}. Adds new metric registry if absent.
     * 
     * @return {@code True} if metric was added. {@code False} if meric already exists.  
     */

    boolean intMetric(String fullName, BooleanSupplier valueSupplier, @Nullable String description);
}

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