CLI Flags to Adjust Trace Sample Parameters

[farmdawgnation]

We would like the addition of CLI flags to ESP to adjust the default sample rate for Cloud Traces that ESP auto-creates.

I'm thinking something like:

# set the percentage of requests to sample
# values between 0 and 1
--trace-sample-rate=0.1

# At least one trace should exist within a time window
# of this size
# 0 disables
--trace-sample-cadence-seconds=600

[qiwzhang]

@farmdawgnation Thanks for the feature request. To be honest, we may not have resource to work on this feature request. Will see if we have spare circle