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

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To keep custom metrics detached from the internals, we should either automaticall add a name prefix to custom name or throw an exception if provided registry name doesn't start with that prefix.

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/**
 * Manages custom metrics. 
 *
 * Metrics have are grouped into registries. Every metric has full name which is conjunction of registry name and aremetric groupedname. into
 registries.* Registry name is part of the full metric name until last '.'. 
 * Metric name is the last part of the full name. Names of custom metrics registers 
 * 
 * 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 thea registry name and "mname" is thea metric name. 
 */
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);

    /** Removes certain registry named '{@code registryName}'  and all its metrics. */
    void removeRegistry(String registryName);


    // Essential metrics. Int, Long and Double should be enough.

    /**
     * Registers a {@link LongMetric}. Adds new metric registry if absent.
     *
     * @return Previous long value setter if it already exists. A new one otherwise.
     */
    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.

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

    /**
     * Registers a {@link DoubleMetric} metric with double 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 int 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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