Log exception data in JSON log

When logging in JSON format, the exception data, e.g. stacktrace, which
the replication plugin logs into its own log are not logged to file.

The implementation was copied from
`com.google.gerrit.pgm.util.ErrorJsonLogEntry` in Gerrit core.

Change-Id: I690b9c21584d98dc69eabaf41fb2d4365f734ce0

src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationLogFile.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationLogFile.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationLogFile.java
index 622034a..7e51f9f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationLogFile.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationLogFile.java
@@ -22,8 +22,11 @@
 import com.google.gerrit.util.logging.JsonLogEntry;
 import com.google.gson.annotations.SerializedName;
 import com.google.inject.Inject;
+import java.util.HashMap;
+import java.util.Map;
 import org.apache.log4j.PatternLayout;
 import org.apache.log4j.spi.LoggingEvent;
+import org.apache.log4j.spi.ThrowableInformation;
 
 public class ReplicationLogFile extends PluginLogFile {
 
@@ -43,13 +46,18 @@
     private class ReplicationJsonLogEntry extends JsonLogEntry {
       public String timestamp;
       public String message;
+      public Map<String, String> exception;
 
       @SerializedName("@version")
-      public final int version = 1;
+      public final int version = 2;
 
       public ReplicationJsonLogEntry(LoggingEvent event) {
         timestamp = timestampFormatter.format(event.getTimeStamp());
         message = (String) event.getMessage();
+
+        if (event.getThrowableInformation() != null) {
+          this.exception = getException(event.getThrowableInformation());
+        }
       }
     }
 
@@ -57,5 +65,25 @@
     public JsonLogEntry toJsonLogEntry(LoggingEvent event) {
       return new ReplicationJsonLogEntry(event);
     }
+
+    private Map<String, String> getException(ThrowableInformation throwable) {
+      HashMap<String, String> exceptionInformation = new HashMap<>();
+
+      String throwableName = throwable.getThrowable().getClass().getCanonicalName();
+      if (throwableName != null) {
+        exceptionInformation.put("exception_class", throwableName);
+      }
+
+      String throwableMessage = throwable.getThrowable().getMessage();
+      if (throwableMessage != null) {
+        exceptionInformation.put("exception_message", throwableMessage);
+      }
+
+      String[] stackTrace = throwable.getThrowableStrRep();
+      if (stackTrace != null) {
+        exceptionInformation.put("stacktrace", String.join("\n", stackTrace));
+      }
+      return exceptionInformation;
+    }
   }
 }