datastore_v3.proto came from: https://github.com/golang/appengine/tree/1956b43a8bbcb66421b5753e14b36ace04469126/internal/datastore
Changes are:
package appengine;
replaced with package luci.gae.internal.datastore;
option go_package
is set to go.chromium.org/luci/gae/service/datastore/internal/protos/datastore