[dynamic control] Add delegating sampler implementation

dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/sampler/DelegatingSampler.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.dynamic.sampler;
+
+import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.api.trace.SpanKind;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.trace.data.LinkData;
+import io.opentelemetry.sdk.trace.samplers.Sampler;
+import io.opentelemetry.sdk.trace.samplers.SamplingResult;
+import java.util.List;
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicReference;
+
+public class DelegatingSampler implements Sampler {
+
+  private final AtomicReference<Sampler> delegate;
+
+  public DelegatingSampler(Sampler initialDelegate) {
+    this.delegate =
+        new AtomicReference<>(initialDelegate != null ? initialDelegate : Sampler.alwaysOn());
+  }
+
+  public DelegatingSampler() {
+    this(Sampler.alwaysOn());
+  }
+
+  public void setDelegate(Sampler sampler) {
+    delegate.set(sampler != null ? sampler : Sampler.alwaysOn());
+  }
+
+  @Override
+  public SamplingResult shouldSample(
+      Context ctx,
+      String traceId,
+      String name,
+      SpanKind kind,
+      Attributes attrs,
+      List<LinkData> links) {
+    return Objects.requireNonNull(delegate.get())
+        .shouldSample(ctx, traceId, name, kind, attrs, links);
+  }
+
+  @Override
+  public String getDescription() {
+    return "delegating/" + Objects.requireNonNull(delegate.get()).getDescription();
+  }
+
+  @Override
+  public String toString() {
+    return getDescription();
+  }
+}