in invoker/core/src/main/java/com/google/cloud/functions/invoker/gcf/JsonLogHandler.java [111:117]
private void appendExecutionId(StringBuilder json, LogRecord record) {
if (executionIdLoggingEnabled()) {
json.append("\"execution_id\": \"")
.append(executionIdByThreadMap.get(Integer.toString(record.getThreadID())))
.append("\", ");
}
}