Add capability to grade the diff of different metrics

for example, if I want to diff submetrics in both metrics PINOT-PERF  and SAR-device, I can do the following in my config file


[PINOT-PERF]
latency.sla=mean>10 p50>5%
drop_rate.sla=mean>0.3 p50>5%

[SAR-device]
sda.tps.sla=mean>50 p50>50%