How to create custom AWS CloudWatch metric for i-node
AWS have published a plugin for collectd which allows to send metrics collected using collectd to CloudWatch.

https://github.com/awslabs/collectd-cloudwatch

The collectd monitoring tool runs on the server and can be configured to collect various metrics, among them metrics on disks and filesystems using their DF plugin. https://collectd.org/wiki/index.php/Plugin:DF

You can turn the ReportInodes option of the CollectD DF plugin and then have these metrics reported to CloudWatch as custom metrics using the AWS CollectD CloudWatch plugin.