Instead, an Istio specific field like `ExpectedTLSIdentity: spiffe://foo.bar` can encode the same information, at a fraction of the cost.

In our testing, even the most generous representations give custom types a 10x edge (in size, allocations, and CPU time) over Envoy types.
In addition, they are more clear and strictly typed; using Envoy types would require us to put a lot of information in untyped `metadata` maps.