We want to make tracing to be opt-in.
For now, we will use `tracing.sample` as the toggle config to enable or
disable tracing and default to sample every traces from uplink cli.
If user wants to change the default sampling rate, they can do so by
using the `--tracing.sample` flag to override the default value
Change-Id: I6f25dac0f43024c50a8aaf6c549e6a514211f834
