Getting Data In

How to extract custom dimensions from plugin_instance when we are using collectd?

According to Getting logs and metrics into metricstore presentation slide 23 - GDI: collectd write_http plugin:

"plugin_instance is currently the only dimension extracted in addition to the default available dimensions"

it means that we can send only one custom dimension not longer than 128 simbols.
We are using Apache Hadoop plugin for collectd to collect apache hadoop metrics.

Could you give any advice how can I extract few dimensions in Splunk from plugin_instance in case i will send a few of them like in sample:
"plugin_instance":"[dim1:val1,dim2:val2,dim3:val3]"