Datastore's ProjectionEntity, IncompleteKey, Entity and Key have static builder methods:

public static Builder builder(ProjectionEntity copyFrom)
public static Builder builder(IncompleteKey copyFrom)
public static Builder builder(Entity copyFrom)
public static Builder builder(Key copyFrom)

In other modules we use a toBuilder() instance method to create builders from entity objects. Should we use toBuilder() also in Datastore? Or is there a reason to prefer static builder(ResourceType copyFrom)? \cc @ajkannan @aozarov