This change adds a README that describes how to export metrics from
Go tools to Prometheus via OpenCensus.
Updates golang/go#33819
Change-Id: Ic417b539fd5ef95fee642321bd30997146cbedb3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/208400
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
