Provide a mechanism for limiting the number of expressions

[lucperkins]

Providing massive expressions is a good way to bring down a server. It's pretty trivial to, for example, write a little script that creates an "or" chain 100,000 Booleans long. Because of this, it'd be good to provide support for optionally limiting the number of expressions evaluated by a Program.

[lucperkins]

I have some ideas on how to make this happen and am happy to do the work. Just wanted to see if I encounter any resistance before I submit a PR 😄

[clarkmcc]

No resistance from me! That's actually something I've been thinking about recently. I know other CEL implementations do something similar to this, i.e. cel-go.

[lucperkins]

@clarkmcc Oh, nice! I was not aware of that implementation, but great to know. Glad you're amenable to this 😄 I'll submit a PR pretty soon here.

[clarkmcc]

Documenting for posterity: this should probably be our guiding star for expression cost estimation/limiting.