Custom Fields might lead to "java.lang.UnsupportedOperationException" with log4j2

[KarstenSchnitter]

Custom fields can be added to log messages by using the CustomField class:

LOGGER.info("Hello World!", CustomField.customField("my_field", "my_value"));

This can result in the following stacktrace:

2022-01-21 09:59:49,158 main ERROR An exception occurred processing Appender Console java.lang.UnsupportedOperationException
	at java.base/java.util.Collections$UnmodifiableMap.put(Collections.java:1457)
	at com.sap.hcp.cf.log4j2.converter.ContextPropsConverter.addCustomFieldsFromArguments(ContextPropsConverter.java:63)
	at com.sap.hcp.cf.log4j2.converter.ContextPropsConverter.format(ContextPropsConverter.java:50)
	at org.apache.logging.log4j.core.pattern.PatternFormatter.format(PatternFormatter.java:44)
	at com.sap.hcp.cf.log4j2.layout.JsonPatternLayout.toSerializable(JsonPatternLayout.java:47)
	at com.sap.hcp.cf.log4j2.layout.JsonPatternLayout.toSerializable(JsonPatternLayout.java:23)
	at org.apache.logging.log4j.core.layout.AbstractStringLayout.toByteArray(AbstractStringLayout.java:282)
	at org.apache.logging.log4j.core.layout.AbstractLayout.encode(AbstractLayout.java:209)
	at org.apache.logging.log4j.core.layout.AbstractLayout.encode(AbstractLayout.java:37)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.directEncodeEvent(AbstractOutputStreamAppender.java:197)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.tryAppend(AbstractOutputStreamAppender.java:190)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:181)
	at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:161)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:134)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:125)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:89)
	at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:542)
	at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:500)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:483)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:417)
	at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:82)
	at org.apache.logging.log4j.core.Logger.log(Logger.java:161)
	at org.apache.logging.log4j.spi.AbstractLogger.tryLogMessage(AbstractLogger.java:2205)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessageTrackRecursion(AbstractLogger.java:2159)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2142)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:2034)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1899)
	at org.apache.logging.slf4j.Log4jLogger.error(Log4jLogger.java:304)

The current implementation of com.sap.hcp.cf.log4j2.converter.ContextPropsConverter.addCustomFieldsFromArgumentsdoes not reflect, that event.getContextData().toMap() might return an unmodifiable map. This needs to be changed to avoid that error.