Define DNS statistic data atom in atoms.proto file

Test: run runtests.sh, make sure no missing test and get all pass
      run /out/host/linux-x86/bin/statsd_testdrive 103
      Got following metric data dump:pass for local test
Bug: 119862317

Change-Id: I691e46ffe4cb30696d35018032d4a7f7628045d9