Double click the "Package.appxmanifest" file in your metro app project, you should see configurations of your app. In the "Application UI" page, you can find "Supported rotations" in the middle. You can check the mode you supported, and leave the "Portrait" mode unchecked. Then you app will not receive any "portrait" mode events anymore.
If you write your own appx manifiest, you can add following block under section, like this
<InitialRotationPreference>
<Rotation Preference="landscape" />
<Rotation Preference="portraitFlipped" />
<Rotation Preference="landscapeFlipped" />
</InitialRotationPreference>