Add default service provider for CorrelationContextManager. (#960)

Signed-off-by: Bogdan Cristian Drutu <bogdandrutu@gmail.com>

api/src/main/java/io/opentelemetry/OpenTelemetry.java

@@ -18,6 +18,7 @@ package io.opentelemetry;
 
 import io.opentelemetry.correlationcontext.CorrelationContextManager;
 import io.opentelemetry.correlationcontext.DefaultCorrelationContextManager;
+import io.opentelemetry.correlationcontext.DefaultCorrelationContextManagerProvider;
 import io.opentelemetry.correlationcontext.spi.CorrelationContextManagerProvider;
 import io.opentelemetry.metrics.DefaultMeterProvider;
 import io.opentelemetry.metrics.DefaultMetricsProvider;
@@ -118,7 +119,7 @@ public final class OpenTelemetry {
     contextManager =
         contextManagerProvider != null
             ? contextManagerProvider.create()
-            : DefaultCorrelationContextManager.getInstance();
+            : DefaultCorrelationContextManagerProvider.getInstance().create();
   }
 
   /**

api/src/main/java/io/opentelemetry/correlationcontext/DefaultCorrelationContextManager.java

@@ -43,7 +43,6 @@ public final class DefaultCorrelationContextManager implements CorrelationContex
    *
    * @return a {@code CorrelationContextManager} singleton that is the default implementation for
    *     {@link CorrelationContextManager}.
-   * @since 0.1.0
    */
   public static CorrelationContextManager getInstance() {
     return INSTANCE;

api/src/main/java/io/opentelemetry/correlationcontext/DefaultCorrelationContextManagerProvider.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2020, OpenTelemetry Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opentelemetry.correlationcontext;
+
+import io.opentelemetry.correlationcontext.spi.CorrelationContextManagerProvider;
+
+public final class DefaultCorrelationContextManagerProvider
+    implements CorrelationContextManagerProvider {
+  private static final CorrelationContextManagerProvider instance =
+      new DefaultCorrelationContextManagerProvider();
+
+  /**
+   * Returns a {@code CorrelationContextManagerProvider} singleton that is the default
+   * implementation for {@link CorrelationContextManager}.
+   *
+   * @return a {@code CorrelationContextManagerProvider} singleton that is the default
+   *     implementation for {@link CorrelationContextManager}.
+   */
+  public static CorrelationContextManagerProvider getInstance() {
+    return instance;
+  }
+
+  @Override
+  public CorrelationContextManager create() {
+    return DefaultCorrelationContextManager.getInstance();
+  }
+
+  private DefaultCorrelationContextManagerProvider() {}
+}

api/src/test/java/io/opentelemetry/OpenTelemetryTest.java

@@ -37,7 +37,7 @@ import io.opentelemetry.metrics.LongObserver;
 import io.opentelemetry.metrics.Meter;
 import io.opentelemetry.metrics.MeterProvider;
 import io.opentelemetry.metrics.spi.MetricsProvider;
-import io.opentelemetry.trace.DefaultTracer;
+import io.opentelemetry.trace.DefaultTracerProvider;
 import io.opentelemetry.trace.Span;
 import io.opentelemetry.trace.SpanContext;
 import io.opentelemetry.trace.Tracer;
@@ -79,17 +79,15 @@ public class OpenTelemetryTest {
 
   @Test
   public void testDefault() {
-    assertThat(OpenTelemetry.getTracerProvider().get("testTracer"))
-        .isInstanceOf(DefaultTracer.getInstance().getClass());
-    assertThat(OpenTelemetry.getTracerProvider().get("testTracer"))
-        .isEqualTo(OpenTelemetry.getTracerProvider().get("testTracer"));
-    assertThat(OpenTelemetry.getMeterProvider())
-        .isInstanceOf(DefaultMeterProvider.getInstance().getClass());
-    assertThat(OpenTelemetry.getMeterProvider()).isEqualTo(OpenTelemetry.getMeterProvider());
+    assertThat(OpenTelemetry.getTracerProvider()).isInstanceOf(DefaultTracerProvider.class);
+    assertThat(OpenTelemetry.getTracerProvider())
+        .isSameInstanceAs(OpenTelemetry.getTracerProvider());
+    assertThat(OpenTelemetry.getMeterProvider()).isInstanceOf(DefaultMeterProvider.class);
+    assertThat(OpenTelemetry.getMeterProvider()).isSameInstanceAs(OpenTelemetry.getMeterProvider());
     assertThat(OpenTelemetry.getCorrelationContextManager())
-        .isInstanceOf(DefaultCorrelationContextManager.getInstance().getClass());
+        .isInstanceOf(DefaultCorrelationContextManager.class);
     assertThat(OpenTelemetry.getCorrelationContextManager())
-        .isEqualTo(OpenTelemetry.getCorrelationContextManager());
+        .isSameInstanceAs(OpenTelemetry.getCorrelationContextManager());
   }
 
   @Test