With this user data script, you can set up cron job to put some custom metrics for CloudWatch at instance initiation.

UserData script for setup CloudWatch

#!/bin/sh
cd /home/ec2-user
wget http://ec2-downloads.s3.amazonaws.com/cloudwatch-samples/CloudWatchMonitoringScripts-v1.1.0.zip
unzip CloudWatchMonitoringScripts-v1.1.0.zip
rm CloudWatchMonitoringScripts-v1.1.0.zip
chown ec2-user:ec2-user aws-scripts-mon
echo "*/5 * * * * ec2-user /home/ec2-user/aws-scripts-mon/mon-put-instance-data.pl --mem-util --disk-space-util --disk-path=/ --from-cron" >> /etc/crontab

It is not working for me.

Instead I write last line as
echo "*/5 * * * * ..." >> /var/spool/cron/ec2-user
to define crob task for ec2-user, or
echo "*/5 * * * * ..." >> /var/spool/cron/root
for root.

Thanks much for the info @Igawin. This works for me. :)
Also for whoever is trying to find related info and ending up here, using /var/spool/cron/ the user shouldn't be included in the syntax as it's already specified as the file name. Otherwise, the daemon will treat it as syntax error and ignore the task.