Change MetricsRegistryImpl constructor to "@VisibleForTesting public".

This enables registry isolation in tests.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173426040

java/google/registry/monitoring/metrics/MetricRegistryImpl.java

@@ -36,7 +36,14 @@ public final class MetricRegistryImpl implements MetricRegistry {
   /** The canonical registry for metrics. The map key is the metric name. */
   private final ConcurrentHashMap<String, Metric<?>> registeredMetrics = new ConcurrentHashMap<>();
 
-  private MetricRegistryImpl() {}
+  
+  /**
+   * Production code must use {@link getDefault}, since this returns the {@link MetricRegistry}
+   * that {@link MetricReporter} uses.  Test code that does not use {@link MetricReporter} can
+   * use this constructor to get an isolated instance of the registry.
+   */
+  @VisibleForTesting
+  public MetricRegistryImpl() {}
 
   public static MetricRegistryImpl getDefault() {
     return INSTANCE;