Hi Raja,
The APM REST API is only designed to access vertex topology data and not metric data.
In the on premise APM Enterprise Manager there is the older Introscope SOAP Web Services API that can be used to read metric data, but that is not available on the APM SaaS/DXI version.
TheSmartstor API for APM Saas enhancement mentioned by Aryne in your other thread APMSQL for CA APM SaaS will also give you this.
Regards,
Lynn