Fix WARN log GELF message has invalid "timestamp" (type: STRING)

Author: slissner

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/GraylogExtendedLogFormatStructuredLogFormatter.java

@@ -123,8 +123,8 @@ private static void jsonMembers(Environment environment, ThrowableProxyConverter
 	 * @param timeStamp the timestamp of the log message
 	 * @return the timestamp formatted as string with millisecond precision
 	 */
-	private static String formatTimeStamp(final long timeStamp) {
-		return new BigDecimal(timeStamp).movePointLeft(3).toPlainString();
+	private static double formatTimeStamp(final long timeStamp) {
+		return new BigDecimal(timeStamp).movePointLeft(3).doubleValue();
 	}
 
 	private static String formatFullMessageWithThrowable(ThrowableProxyConverter throwableProxyConverter,

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/logback/GraylogExtendedLogFormatStructuredLogFormatterTests.java

@@ -59,7 +59,7 @@ void shouldFormat() {
 		assertThat(json).endsWith("\n");
 		Map<String, Object> deserialized = deserialize(json);
 		assertThat(deserialized).containsExactlyInAnyOrderEntriesOf(map("version", "1.1", "host", "name", "timestamp",
-				"1719910193.000", "level", 6, "_level_name", "INFO", "_process_pid", 1, "_process_thread_name", "main",
+				1719910193.000D, "level", 6, "_level_name", "INFO", "_process_pid", 1, "_process_thread_name", "main",
 				"_service_version", "1.0.0", "_service_environment", "test", "_service_node_name", "node-1",
 				"_log_logger", "org.example.Test", "short_message", "message", "_mdc-1", "mdc-v-1", "_kv-1", "kv-v-1"));
 	}