In the Dynatrace supported technologies and even on forum haven't found anything related to monitoring of HP 3par device metrics either out of the box or even using Plugin to pull Custom Metrics realted to storage. Has someone done anything similar?
Not directly out-of-box. Since it's an appliance, the relevant approach would be to use ActiveGate plugin to collect metrics. I'm not aware of any current plugin that would directly monitor it, but if you can use SNMP to poll metrics from the appliance, probably the easiest way would be to use the SNMP plugin and fork and extend it to your needs (your metrics).