Is your environment managed by Apigee, or yourself? If it's a managed solution that you dont have access to, you would need to work with their support team to have forwarders enabled and the relevant apps installed.
If you have this locally and manage it yourself, it's pretty straight forward. You install the forwarders on your Apigee nodes, and then use the Splunk JMX connector to pull the data out.
See this Splunk Add-On : https://splunkbase.splunk.com/app/2647/