Adjust GcpLayout JSON to latest format

First, it formats the log timestamp field to the correct format
recognized by Fluent-Bit (component of Google Cloud Logging) and Google
Ops Agent.

Secondly, severity field now must be prefixed with
logging.googleapis.com.

Third, counter cannot be used for insertId as it is duplicated on
different threads.

And the last but not the least, exception, thread and logger fields are
pretty standard when logging via Logback's JSON layout and Google's
Spring GCP libraries. Field name changes now match these other loggers.

Author: ViliusS

log4j-layout-template-json-test/src/test/java/org/apache/logging/log4j/layout/template/json/GcpLayoutTest.java

@@ -149,40 +149,28 @@ private static void verifySerialization(final LogEvent logEvent) {
             }
 
             // Verify insert id.
-            assertThat(accessor.getString("logging.googleapis.com/insertId")).matches("[-]?[0-9]+");
+            assertThat(accessor.getString("logging.googleapis.com/insertId")).matches("[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}");
 
             // Verify exception.
             if (exception != null) {
 
-                // Verify exception class.
-                assertThat(accessor.getString(new String[] {"_exception", "class"}))
-                        .isEqualTo(exception.getClass().getCanonicalName());
-
-                // Verify exception message.
-                assertThat(accessor.getString(new String[] {"_exception", "message"}))
-                        .isEqualTo(exception.getMessage());
-
                 // Verify exception stack trace.
-                assertThat(accessor.getString(new String[] {"_exception", "stackTrace"}))
+                assertThat(accessor.getString("exception"))
                         .contains(exception.getLocalizedMessage())
                         .contains("at org.apache.logging.log4j.layout.template.json")
                         .contains("at " + JAVA_BASE_PREFIX + "java.lang.reflect.Method")
                         .contains("at org.junit.platform.engine");
 
             } else {
-                assertThat(accessor.getObject(new String[] {"_exception", "class"}))
-                        .isNull();
-                assertThat(accessor.getObject(new String[] {"_exception", "message"}))
-                        .isNull();
-                assertThat(accessor.getString(new String[] {"_exception", "stackTrace"}))
+                assertThat(accessor.getString("exception"))
                         .isEmpty();
             }
 
             // Verify thread name.
-            assertThat(accessor.getString("_thread")).isEqualTo(logEvent.getThreadName());
+            assertThat(accessor.getString("thread")).isEqualTo(logEvent.getThreadName());
 
             // Verify logger name.
-            assertThat(accessor.getString("_logger")).isEqualTo(logEvent.getLoggerName());
+            assertThat(accessor.getString("logger")).isEqualTo(logEvent.getLoggerName());
         });
     }
 

log4j-layout-template-json/src/main/resources/GcpLayout.json

@@ -1,13 +1,18 @@
 {
-  "timestamp": {
+  "timestampSeconds": {
     "$resolver": "timestamp",
-    "pattern": {
-      "format": "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'",
-      "timeZone": "UTC",
-      "locale": "en_US"
+    "epoch": {
+      "unit": "secs",
+      "rounded": true
     }
   },
-  "severity": {
+  "timestampNanos": {
+    "$resolver": "timestamp",
+    "epoch": {
+      "unit": "secs.nanos"
+    }
+  },
+  "logging.googleapis.com/severity": {
     "$resolver": "pattern",
     "pattern": "%level{WARN=WARNING, TRACE=DEBUG, FATAL=EMERGENCY}",
     "stackTraceEnabled": false
@@ -37,8 +42,9 @@
     }
   },
   "logging.googleapis.com/insertId": {
-    "$resolver": "counter",
-    "stringified": true
+    "$resolver": "pattern",
+    "pattern": "%uuid{TIME}",
+    "stackTraceEnabled": false
   },
   "logging.googleapis.com/trace": {
     "$resolver": "mdc",
@@ -49,25 +55,15 @@
     "key": "span_id"
   },
   "logging.googleapis.com/trace_sampled": true,
-  "_exception": {
-    "class": {
-      "$resolver": "exception",
-      "field": "className"
-    },
-    "message": {
-      "$resolver": "exception",
-      "field": "message"
-    },
-    "stackTrace": {
-      "$resolver": "pattern",
-      "pattern": "%xEx"
-    }
+  "exception": {
+    "$resolver": "pattern",
+    "pattern": "%xEx"
   },
-  "_thread": {
+  "thread": {
     "$resolver": "thread",
     "field": "name"
   },
-  "_logger": {
+  "logger": {
     "$resolver": "logger",
     "field": "name"
   }