Add TraceState.asMap (#2190)

* Add TraceState.asMap

* Javadoc

* Spot

api/src/main/java/io/opentelemetry/api/internal/ReadOnlyArrayMap.java

@@ -30,10 +30,12 @@ import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Set;
 
+/** A read-only view of an array of key-value pairs. */
 @SuppressWarnings("unchecked")
-final class ReadOnlyArrayMap<K, V> implements Map<K, V> {
+public final class ReadOnlyArrayMap<K, V> implements Map<K, V> {
 
-  static <K, V> Map<K, V> wrap(List<Object> array) {
+  /** Returns a read-only view of the given {@code array}. */
+  public static <K, V> Map<K, V> wrap(List<Object> array) {
     if (array.isEmpty()) {
       return Collections.emptyMap();
     }

api/src/main/java/io/opentelemetry/api/trace/ArrayBasedTraceState.java

@@ -6,8 +6,10 @@
 package io.opentelemetry.api.trace;
 
 import com.google.auto.value.AutoValue;
+import io.opentelemetry.api.internal.ReadOnlyArrayMap;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.function.BiConsumer;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
@@ -46,6 +48,12 @@ abstract class ArrayBasedTraceState implements TraceState {
     }
   }
 
+  @Override
+  @SuppressWarnings({"unchecked", "rawtypes"})
+  public Map<String, String> asMap() {
+    return ReadOnlyArrayMap.wrap((List) getEntries());
+  }
+
   abstract List<String> getEntries();
 
   @Override

api/src/main/java/io/opentelemetry/api/trace/TraceState.java

@@ -5,6 +5,7 @@
 
 package io.opentelemetry.api.trace;
 
+import java.util.Map;
 import java.util.function.BiConsumer;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
@@ -69,6 +70,9 @@ public interface TraceState {
   /** Iterates over all the key-value entries contained in this {@link TraceState}. */
   void forEach(BiConsumer<String, String> consumer);
 
+  /** Returns a read-only view of this {@link TraceState} as a {@link Map}. */
+  Map<String, String> asMap();
+
   /**
    * Returns a {@code Builder} based on this {@code TraceState}.
    *

api/src/test/java/io/opentelemetry/api/trace/TraceStateTest.java

@@ -64,6 +64,17 @@ class TraceStateTest {
         .containsExactly(entry(SECOND_KEY, SECOND_VALUE), entry(FIRST_KEY, FIRST_VALUE));
   }
 
+  @Test
+  void asMap() {
+    assertThat(firstTraceState.asMap()).containsExactly(entry(FIRST_KEY, FIRST_VALUE));
+
+    assertThat(secondTraceState.asMap()).containsExactly(entry(SECOND_KEY, SECOND_VALUE));
+
+    // Reverse order of input.
+    assertThat(multiValueTraceState.asMap())
+        .containsExactly(entry(SECOND_KEY, SECOND_VALUE), entry(FIRST_KEY, FIRST_VALUE));
+  }
+
   @Test
   void disallowsNullKey() {
     assertThat(EMPTY.toBuilder().set(null, FIRST_VALUE).build()).isEqualTo(EMPTY);