wiki: publish some benchmarks

[odeke-em]

Firstly, thank you for the great work on OpenCensus-CPP!

I kindly would like to ask if perhaps someone could run and publish some benchmarks on the wiki. I ask because I have been talking to companies and almost all their tech and developers would like to know what the overhead is when traced vs untraced. I believe the wiki would be a great place to publish such values and those could be updated by anyone, without having to make a pull request or distract the folks watching and maintaining this repo.