...

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

...

/**
* 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

...

Interfaces of writeable

...

metrics

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

...

Names like "LongMetric" or "ObjectMetric" we already have in the package "org.apache.ignite.spi.metric".

3.4 Examples of

...

writeable metric intefraces

package org.apache.ignite.metric;

...

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

package org.apache.ignite;

...

    /**
    * Adds a long value custom metric.
    *
    * @return New or previously registered long value metric. {@code Null} if previous metric is not a {@link LongConsumer}.
    */
    @Nullable LongConsumer longMetric(String registryName, String metricName, @Nullable String description);

    /**
    * Adds a double value custom metric.
    *
    * @return New or previously registered long value metric. {@code Null} if previous metric is not a {@link DoubleConsumer}.
    */
    @Nullable DoubleConsumer doubleMetric(String registryName, String metricName, @Nullable String description);

    /**
    * Adds a int value custom metric.
    *
    * @return New or previously registered long value metric. {@code Null} if previous metric is not an {@link IntConsumer}.
    */
    @Nullable IntConsumer booleanMetric(String registryName, String metricName, @Nullable String description);

    /**
    * Adds a long value custom metric.
    *
    * @return {@code True} if {@code supplier} has been registered as a new int metric. {@code False}, if a previous
    * value supplier already exists.
    */
    boolean longMetric(String registryName, String metricName, LongSupplier supplier, @Nullable String description);

    /**
    * Adds a double value custom metric.
    *
    * @return {@code True} if {@code supplier} has been registered as a new int metric. {@code False}, if a previous
    * value supplier already exists.
    */
    boolean doubleMetric(String registryName, String metricName, DoubleSupplier supplier, @Nullable String description);

    /**
    * Adds a int value custom metric.
    *
    * @return {@code True} if {@code supplier} has been registered as a new int metric. {@code False}, if a previous
    * value supplier already exists.
    */
    boolean intMetric(String registryName, String metricName, BooleanSupplier supplier, @Nullable String description);

    /** Removes certain custom metric. */
    void removeCustomMetric(String registryName, String metricName);

    /** Removes entire custom metric registry. */
    void removeCustomRegistry(String registryName);

    /** Provides custom or internal read-only metric registry. */
    @Nullable ReadOnlyMetricRegistry findRegistry(String registryName);
}

...

Code example

Custom metric within a service.

/** */
public static final class TestCustomMetricsService implements TestService {
    /** */
    @IgniteInstanceResource
    private Ignite ignite;

    /** */
    @ServiceContextResource
    private ServiceContext ctx;

    /** */
    private AtomicReference<UUID> remoteId;

    /** */
    private final AtomicInteger metricValue = new AtomicInteger();

    /** {@inheritDoc} */
    @Override public void init() throws Exception {
        remoteId = new AtomicReference<>();

        ignite.metrics().customRegistry(regName(ctx.name())).gauge("filteredInvocation", metricValue::get, "Counter of speceific service invocation.");

        ignite.metrics().customRegistry(regName(ctx.name())).gauge(LOAD_THRESHOLD_METRIC_NAME, () -> metricValue.get() >= 100, "Load flag.");

        ignite.metrics().customRegistry(regName(ctx.name())).gauge("loaded", () -> remoteId.get(), UUID.class, "Remote system class id.");
}

    /** {@inheritDoc} */
    @Override public void cancel() {
        refresh();

        ignite.metrics().customRegistry(regName(ctx.name())).remove(COUNTER_METRIC_NAME);
    }

    /** {@inheritDoc} */
    @Override public void refresh() {
        metricValue.set(0);

        remoteId.set(null);
    }

    /** */
    @Override public void invoke(int param) {
        if (ctx.isCancelled())
            return;

        remoteId.compareAndSet(null, UUID.randomUUID());

        // Updates metric sometimes.
        if (!ctx.isCancelled() && param % 10 == 0)
            metricValue.set(param / 10);
    }

    /** */
    private static String regName(String svcName) {
        return "service." + svcName;
    }
}

Custom metrics within a computation

/** */
private static final class TestCustomMetricsComputeTask extends ComputeTaskAdapter<Void, Long> {
    /** */
    private static final class TestComputeJob extends ComputeJobAdapter {
        /** Ignite instance. */
        @IgniteInstanceResource
        private Ignite ignite;

        /** {@inheritDoc} */
        @Override public Long execute() throws IgniteException {
            long val = 0;

            // Some job limit.
            long limit = 300 + ThreadLocalRandom.current().nextLong(700);

            LongValueMetric metricCur = ignite.metrics().customRegistry(METRIC_REGISTRY).longMetric("task.test", null);
            LongSumMetric metricTotal = ignite.metrics().customRegistry(METRIC_REGISTRY).longAdderMetric("total", null);
            LongSumMetric metricTicks = ignite.metrics().customRegistry(METRIC_REGISTRY).longAdderMetric("ticks", null);

            while (!isCancelled() && val < limit) {
                // Does some job.
                try {
                    U.sleep(ThreadLocalRandom.current().nextInt(50));
                }
                catch (IgniteInterruptedCheckedException ignored) {
                    //No op.
                }

                long increment = ThreadLocalRandom.current().nextLong(100);

                val += increment;

                metricTicks.increment()

            }

            metricCur.value(val);

            metricTotal.add(val);

            return isCancelled() ? 0 : val;
        }
    }
}

Further Steps

We already have implementations of more complex and useful metrics. We could also store custom metrics. Thus, the development stages might be:

...