The location where android trace files are stored on Nexus 5X with Android 7.0 is

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/sdcard/Android/data/com.domain.myapp/files/filename.trace

After using the os.Debug class to generate trace files for an app

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Debug.startMethodTracing("mytracefile");
Debug.stopMethodTracing();

the file can be pulled from the stored location like

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adb pull /sdcard/Android/data/com.domain.myapp/files/mytracefile.trace