Add support for structured logging

Update Logback and Log4j2 integrations to support structured logging.
Support for the ECS and Logstash JSON formats is provided out-of-the-box
and the `StructuredLogFormatter` interface may be used to if further
custom formats need to be supported.

Closes gh-5479

Co-authored-by: Phillip Webb <phil.webb@broadcom.com>

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/logging/structured/customformat/MyCustomFormat.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2012-2024 the original author or 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
+ *
+ *      https://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 org.springframework.boot.docs.features.logging.structured.customformat;
+
+import ch.qos.logback.classic.spi.ILoggingEvent;
+
+import org.springframework.boot.logging.structured.StructuredLogFormatter;
+
+class MyCustomFormat implements StructuredLogFormatter<ILoggingEvent> {
+
+	@Override
+	public String format(ILoggingEvent event) {
+		return "time=" + event.getInstant() + " level=" + event.getLevel() + " message=" + event.getMessage() + "\n";
+	}
+
+}