metrics_atoms_protoc_plugin: Fix pylint errors am: cdee6e0a4e am: cf20d1b73b

Original change: https://android-review.googlesource.com/c/trusty/host/common/+/2757187

Change-Id: Ie9f2731c528e251541aaed223c13cd703ca6c424
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>