add key replacement

Author: JoeWang1127

java-sdk-logging/logback-extension/src/main/java/com/google/api/logging/SDKLoggingMdcJsonProvider.java

@@ -65,6 +65,7 @@ public void writeTo(JsonGenerator generator, ILoggingEvent event) throws IOExcep
         continue;
       }
 
+      fieldName = getMdcKeyFieldNames().getOrDefault(entry.getKey(), fieldName);
       if (!hasWrittenStart && getFieldName() != null) {
         generator.writeObjectFieldStart(getFieldName());
         hasWrittenStart = true;

java-sdk-logging/logback-extension/src/test/java/com/google/api/logging/SDKLoggingMdcJsonProviderTest.java

@@ -173,6 +173,25 @@ void testWriteToJsonTreeIncludedKey() throws IOException {
     verify(generator, never()).writeObject(anyString());
   }
 
+  @Test
+  void testWriteToJsonTreeReplacedKey() throws IOException {
+    mdc.put(
+        "json1",
+        "{\n"
+            + "  \"@version\": \"1\",\n"
+            + "  \"textPayload\": \"Received response\",\n"
+            + "  \"response.payload\": {\n"
+            + "    \"name\": \"example\",\n"
+            + "    \"state\": \"ACTIVE\"\n"
+            + "  }\n"
+            + "}");
+    provider.addMdcKeyFieldName("json1=new_json");
+    provider.writeTo(generator, event);
+    verify(generator, times(1)).writeFieldName("new_json");
+    verify(generator, times(1)).writeTree(any(JsonNode.class));
+    verify(generator, never()).writeObject(anyString());
+  }
+
   @Test
   void testWriteToJsonTreeExcludedKey() throws IOException {
     mdc.put(