Switch flag definitions to be on pflag instead of flag in `package go/stats/statsd`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/stats/statsd/*.go
go/stats/statsd/statsd.go:      statsdAddress    = flag.String("statsd_address", "", "Address for statsd client")
go/stats/statsd/statsd.go:      statsdSampleRate = flag.Float64("statsd_sample_rate", 1.0, "")