code: package io.ray.runtime.metric;
  
  import com.google.common.base.Preconditions;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  
  /** The entry of metrics for easy use. */
  public final class Metrics {
  
    private static MetricRegistry metricRegistry;
  
    public static MetricRegistry init(MetricConfig metricConfig) {
      synchronized (Metrics.class) {
        metricRegistry = new MetricRegistry();
        metricRegistry.startup(metricConfig);
        return metricRegistry;
      }
    }
  
    public static void shutdown() {
      synchronized (Metrics.class) {
        if (metricRegistry != null) {
          metricRegistry.shutdown();
          metricRegistry = null;
        }
      }
    }
  
    public static CountBuilder count() {
      return new CountBuilder();
    }
  
    public static GaugeBuilder gauge() {
      return new GaugeBuilder();
    }
  
    public static SumBuilder sum() {
      return new SumBuilder();
    }
  
    public static HistogramBuilder histogram() {
      return new HistogramBuilder();
    }
  
    public static class CountBuilder extends AbstractBuilder<CountBuilder, Count> {
  
      @Override
      protected Count create() {
        return new Count(name, description, unit, generateTagKeysMap(tags));
      }
    }
  
    public static class GaugeBuilder extends AbstractBuilder<GaugeBuilder, Gauge> {
  
      @Override
      protected Gauge create() {
        return new Gauge(name, description, unit, generateTagKeysMap(tags));
      }
    }
  
    public static class SumBuilder extends AbstractBuilder<SumBuilder, Sum> {
  
      @Override
      protected Sum create() {
        return new Sum(name, description, unit, generateTagKeysMap(tags));
      }
    }
  
    public static class HistogramBuilder extends AbstractBuilder<HistogramBuilder, Histogram> {
  
      private List<Double> boundaries;
  
      public HistogramBuilder boundaries(List<Double> boundaries) {
        this.boundaries = boundaries;
        return this;
      }
  
      @Override
      protected Histogram create() {
        return new Histogram(name, description, unit, boundaries, generateTagKeysMap(tags));
      }
    }
  
    public abstract static class AbstractBuilder<B extends AbstractBuilder, M extends Metric> {
      protected String name;
      protected String description;
      protected String unit;
      protected Map<String, String> tags;
  
      public B name(String name) {
        this.name = Preconditions.checkNotNull(name);
        return (B) this;
      }
  
      public B description(String description) {
        this.description = Preconditions.checkNotNull(description);
        return (B) this;
      }
  
      public B unit(String unit) {
        this.unit = Preconditions.checkNotNull(unit);
        return (B) this;
      }
  
      public B tags(Map<String, String> tags) {
        this.tags = Preconditions.checkNotNull(tags);
        return (B) this;
      }
  
      /**
       * Creates a metric by sub-class.
       *
       * @return a metric
       */
      protected abstract M create();
  
      public M register() {
        M m = create();
        maybeInitRegistry();
        return (M) metricRegistry.register(m);
      }
    }
  
    private static Map<TagKey, String> generateTagKeysMap(Map<String, String> tags) {
      Map<TagKey, String> tagKeys = new HashMap<>(tags.size() * 2);
      tags.forEach(
          (key, value) -> {
            TagKey tagKey = new TagKey(key);
            tagKeys.put(tagKey, value);
          });
      return tagKeys;
    }
  
    private static MetricRegistry maybeInitRegistry() {
      synchronized (Metrics.class) {
        if (metricRegistry != null) {
          return metricRegistry;
        } else {
          metricRegistry = MetricRegistry.DEFAULT_REGISTRY;
          metricRegistry.startup();
          return metricRegistry;
        }
      }
    }
  }
  