...

• Refactor a bit current metrics. Create interfaces for writable metric registry, metric manager. Create interfaces for some essential metrics (like int, double, long, longAdder, boolean, object ) and the gauge metrics (IntGauge, LongGauge, etc.).  are below.
• Provide tiny enough facade with consumers and suppliers for int, long, double value metrics.

...

   public IgniteMetrics metrics();

}

3.  Interfaces for existing metrics.

Instead of #4, we could create interfaces for the existing metrics implementations.

3.1 IgniteMetric

package org.apache.ignite.metric;

/**
 * Allows to manage custom metrics and to obtain read-only internal metrics.
 *
 * Note: Names of custom metric registries are required to start with 'custom.' (lower case) and may have additional
 * dot-separated qualifiers. The prefix is automatically added if missed. For example, if provided custom registry name 
 * is "a.b.c.mname", it is automatically extended to "custom.a.b.c.mname".
 * 
 * Any custom name or dot-separated name part cannot have spaces and must not be empty. Spaces are removed.
 * 
 * Examples of custom metric registry names: "custom", "custom.admin", "custom.admin.sessions", "custom.processes", etc.
 *
 * @see ReadOnlyMetricRegistry
 * @see IgniteMetricRegistry
 */
@IgniteExperimental
public interface IgniteMetrics extends Iterable<ReadOnlyMetricRegistry> {
    /**
     * Gets or creates custom metric registry named "custom." + {@code registryName}.
     *
     * @return {@link IgniteMetricRegistry} registry.
     */
    IgniteMetricRegistry customRegistry(String registryName);

    /**
     * Gets or creates custom metric registry named "custom.".
     *
     * @return {@link IgniteMetricRegistry} registry.
     */
    default IgniteMetricRegistry customRegistry() {
        return customRegistry(null);
    }

    /**
     * Gets metric registry including the Ignite's internal registries.
     *
     * @return Certain read-only metric registry.
     */
    @Nullable ReadOnlyMetricRegistry findRegistry(String registryName);

    /** */
    void removeCustomRegistry(String registryName);

    /** Removes custom metric registry with name '.custom'./
    default void removeCustomRegistry() {
        removeCustomRegistry(null);
    }
}

3.2 IgniteMetricRegistry

Probably should be named "MetricRegistry" with renaming of internal "MetricRegistry" to "MetricRegistryImpl".  

package org.apache.ignite.metric;

/**
 * Metric registry. Allows to get, add or remove metrics.
 *
 * @see IgniteMetrics
 * @see ReadOnlyMetricRegistry
 */
@IgniteExperimental
public interface IgniteMetricRegistry extends ReadOnlyMetricRegistry {
     /**
     * Registers an int metric which value will be queried from the specified supplier.
     *
     * @return {@code True} if new metric was added. {@code False} is other metric already exists with the same name.
     */
    boolean gauge(String name, IntSupplier supplier, @Nullable String desc);

    /**
     * Registers a long which value will be queried from the specified supplier.
     *
     * @return {@code True} if new metric was added. {@code False} is other metric already exists with the same name.
     */
    boolean gauge(String name, LongSupplier supplier, @Nullable String desc);

    /**
     * Registers a double metric which value will be queried from the specified supplier.
     *
     * @return {@code True} if new metric was added. {@code False} is other metric already exists with the same name.
     */
    boolean gauge(String name, DoubleSupplier supplier, @Nullable String desc);

    /**
     * Registers an object metric which value will be queried from the specified supplier.
     *
     * @return {@code True} if new metric was added. {@code False} is other metric already exists with the same name.
     */
    <T> boolean gauge(String name, Supplier<T> supplier, Class<T> type, @Nullable String desc);

    /**
     * Registers a boolean metric which value will be queried from the specified supplier.
     *
     * @return {@code True} if new metric was added. {@code False} is other metric already exists with the same name.
     */
    boolean gauge(String name, BooleanSupplier supplier, @Nullable String desc);

    /**
     * Registers an updatable int metric.
     *
     * @return New {@link IntValueMetric} or previous one with the same name. {@code Null} if previous metric exists and
     * is not a {@link IntValueMetric}.
     */
    @Nullable IntValueMetric intMetric(String name, @Nullable String desc);

    /**
     * Registers an updatable long metric.
     *
     * @return New {@link LongValueMetric} or previous one with the same name. {@code Null} if previous metric exists and
     * is not a {@link LongValueMetric}.
     */
    @Nullable LongValueMetric longMetric(String name, @Nullable String desc);

    /**
     * Registers an updatable long adder metric.
     *
     * @return New {@link LongValueMetric} or previous one with the same name. {@code Null} if previous metric exists and
     * is not a {@link LongValueMetric}.
     */
    @Nullable LongSumMetric longAdderMetric(String name, @Nullable String desc);

    /**
     * Registers an updatable double metric.
     *
     * @return New {@link DoubleValueMetric} or previous one with the same name. {@code Null} if previous metric exists and
     * is not a {@link DoubleValueMetric}.
     */
    @Nullable DoubleValueMetric doubleMetric(String name, @Nullable String desc);

    /**
     * Registers an updatable double metric.
     *
     * @param <T> Metric value type.
     * @return New {@link CustomMetric} or previous one with the same name. {@code Null} if previous metric exists and
     * is not a {@link CustomMetric}.
     */
    @Nullable <T> AnyValueMetric<T> objectMetric(String name, Class<T> type, @Nullable String desc);

    /**
     * Removes metrics with the {@code name}.
     *
     * @param name Metric name..
     */
    void remove(String name);

    /** Resets all metrics of this metric registry. */
    void reset();
}

3.3 New writeable metric intefraces

To the package "org.apache.ignite.metric" we add:

• BooleanValueMetric

  IntValueMetric

  LongValueMetric

  LongSumMetric (a long adder)

  DoubleValueMetric

  AnyValueMetric<T> (an object metric)

4. Minimal Custom Metric interface

Instead of #3, we could bring only a minimal interface.

package org.apache.ignite;

...

1. IEP-35 Monitoring & Profiling
2. New Metric System
3. Ticket of a public metric API
4. IEP-116 : Ignite 3 metric

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