Add trace spans for client

[douglas-reid]

For the distributed tracing features of Istio, we want to have mixerclient spans in the generated trace data. This would help with a number of debug-ability related items:

• better understanding caching
• better understanding perf of client library
• better understanding impact of network

cc: @rshriram

[geeknoid]

Issue moved to istio/proxy #1193 via ZenHub