annotations.proto was copied from https://chromium.googlesource.com/infra/luci/luci-go/+/a6a8e58d4ee99046883251e60d04733dcbda6d04/common/proto/milo/annotations.proto and modified to get rid of manifest.proto import because it causes go.chromium.org.luci.common.proto.milo.manifest Python import.