[Metrics] Example for filtering which metrics we want to export

[bsarden]

I'm trying to build a user application that exposes which metrics we want to export via regex. For example:

# Export all metrics under the foo.bar namespace
$ ./my-application --metrics_filter="foo\.bar.*"

Is this possible to do? IIUC, the InstrumentSelector, MetricSelector, and Predicate classes indicate that this functionality is achievable. However, I can't find any pointers into how we expose the ability to configure which metrics to export to the user.