What is a good simple metric for a single photon?
True, but given any metric you can compute the Einstein tensor which is proportional to the stress energy tensor. Thus the OP question can be taken as what is an example of metric producing the stress energy tensor of a photon. Then the answer is the one Pervect gave - there is none because stress energy tensor is a classical construct.What do you mean by "metric for a photon"? The metric is a property of spacetime, not of its contents.