doc: update metrics and artifacts docs
doc: update metrics and artifacts docs

Update documentation for metrics and artifacts documentation to show
how these may be added by a plugin.

setrofim/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,setrofim/workload-automation,lisatn/workload-automation,setrofim/workload-automation,lisatn/workload-automation,lisatn/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,lisatn/workload-automation