add "get" method to DeclarativeConfigProperties

Author: zeitlinger

api/incubator/src/main/java/io/opentelemetry/api/incubator/config/DeclarativeConfigProperties.java

@@ -201,6 +201,17 @@ default DeclarativeConfigProperties getStructured(
     return defaultIfNull(getStructured(name), defaultValue);
   }
 
+  /**
+   * Returns a {@link DeclarativeConfigProperties} configuration property.
+   *
+   * @return a map-valued configuration property, or an empty {@link DeclarativeConfigProperties}
+   *     instance if {@code name} has not been configured
+   * @throws DeclarativeConfigException if the property is not a mapping
+   */
+  default DeclarativeConfigProperties get(String name) {
+    return defaultIfNull(getStructured(name), empty());
+  }
+
   /**
    * Returns a list of {@link DeclarativeConfigProperties} configuration property.
    *