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
yum install -y perl-Switch perl-DateTime perl-Sys-Syslog perl-LWP-Protocol-https perl-Digest-SHA unzip
cd /home/ec2-user
curl http://aws-cloudwatch.s3.amazonaws.com/downloads/CloudWatchMonitoringScripts-1.2.1.zip -O
unzip CloudWatchMonitoringScripts-1.2.1.zip
rm -rf CloudWatchMonitoringScripts-1.2.1.zip
chown ec2-user:ec2-user aws-scripts-mon
echo "* * * * * /home/ec2-user/aws-scripts-mon/mon-put-instance-data.pl --mem-util --swap-util --aggregated --auto-scaling --from-cron" >> /var/spool/cron/ec2-user
echo "0 * * * * /home/ec2-user/aws-scripts-mon/mon-put-instance-data.pl --disk-space-used --disk-space-avail --disk-space-util --disk-path=/  --aggregated --auto-scaling --from-cron" >> /var/spool/cron/ec2-user