Automatically calculate and display network latency for a span in the UI

[mansu]

Currently, when we look at a span belonging to a trace in the UI, one has to mentally calculate the network latency to identify what percentage of the latency came from processing and what percentage of latency came from request processing.

So, if a span has cs, cr, ss and sr, the network latency would be (sr - cs) + (ss - cr). Bonus points if the network latency can be shown like the chrome network latency tab. Adding an image from chrome documentation for reference.

[codefromthecrypt]

This issue was moved to openzipkin-attic/zipkin-ui#16

[codefromthecrypt]

moved because there are similar concerns in the zipkin-ui repo