Address latest PR feedback

- Wrap return value in Collections.unmodifiableMap() to prevent external mutation
- Update license headers for new files to use the new format
- Throw IllegalStateException instead of returning null in get() method

Signed-off-by: Manuel Mangas Zurita <mzurita@amazon.com>

Author: mzurita-amz

data-prepper-core/src/main/java/org/opensearch/dataprepper/core/parser/config/MetricsConfig.java

@@ -192,12 +192,12 @@ private EMFLoggingRegistryConfig createEMFLoggingRegistryConfig(final DataPreppe
         return new EMFLoggingRegistryConfig() {
             @Override
             public String get(String key) {
-                return null;
+                throw new IllegalStateException("This method should never be called under normal circumstances");
             }
 
             @Override
             public Map<String, String> additionalProperties() {
-                return dataPrepperConfiguration.getEmfAdditionalProperties();
+                return Collections.unmodifiableMap(dataPrepperConfiguration.getEmfAdditionalProperties());
             }
         };
     }

data-prepper-core/src/main/java/org/opensearch/dataprepper/core/parser/model/EmfConfig.java

@@ -1,6 +1,11 @@
 /*
  * Copyright OpenSearch Contributors
  * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ *
  */
 
 package org.opensearch.dataprepper.core.parser.model;

data-prepper-core/src/test/java/org/opensearch/dataprepper/core/parser/model/EmfConfigTest.java

@@ -1,6 +1,11 @@
 /*
  * Copyright OpenSearch Contributors
  * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ *
  */
 
 package org.opensearch.dataprepper.core.parser.model;