Add one field (sampling_rate_denom) in NetworkDnsEventReported

The sample rate of DNS stats(to statsd) is 1/sampling_rate_denom.

Bug: 133111380
Test: ./out/host/linux-x86/bin/statsd_testdrive 116
Change-Id: Ibb8edd6b0dfc14b55c7c1b8d84877297516fb55c
2 files changed