Expose Context.empty() as building block for creating custom root contexts (#8575)

This supports custom ContextManagers that want to define their own root context without needing to expose all the different context implementations. They can start with Context.empty() and add any elements they want, storing the final context as a constant so it can be supplied as Context.root()

Author: mcculls

components/context/src/main/java/datadog/context/Context.java

@@ -36,6 +36,15 @@
  * @see ContextKey
  */
 public interface Context {
+  /**
+   * Returns the empty context.
+   *
+   * @return the context containing no values at all.
+   */
+  static Context empty() {
+    return EmptyContext.INSTANCE;
+  }
+
   /**
    * Returns the root context.
    *

components/context/src/test/java/datadog/context/ContextTest.java

@@ -1,5 +1,6 @@
 package datadog.context;
 
+import static datadog.context.Context.empty;
 import static datadog.context.Context.root;
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -21,6 +22,17 @@ class ContextTest {
   static final ContextKey<Float> FLOAT_KEY = ContextKey.named("float-key");
   static final ContextKey<Long> LONG_KEY = ContextKey.named("long-key");
 
+  @Test
+  void testEmpty() {
+    // Test empty is always the same
+    Context empty = empty();
+    assertEquals(empty, empty(), "Empty context should be consistent");
+    // Test empty is not mutated
+    String stringValue = "value";
+    empty.with(STRING_KEY, stringValue);
+    assertEquals(empty, empty(), "Empty context should be immutable");
+  }
+
   @Test
   void testRoot() {
     // Test root is always the same