New CEL convenience function: random()

[wtzhang23]

Title: New CEL convenience function: random()

Description:

• When called, use Random::RandomGenerator to generate a random uint64
• Useful in Access Logger Filter to introduce sampling
• Although this is possible with the Runtime filter, this would be useful for end users

Questions:

• Should this convenience function be available everywhere? E.g. the access log formatter?
• On the flip note, should this function, and others in the future, be explicitly requested by the user? E.g. a list of extensions provided with a cel expression under a new type:

  
  message CelActivationSettings {
  repeated google.protobuf.Any typed_functions = 1;
  }

message CelRandomFunction { }

...

[tyxia]

@wtzhang23

Is this issue for sampling in access log OR for general random purpose? This is ongoing effort https://github.com/envoyproxy/envoy/pull/34611 for former.

Also, that will be great to introduce random feature in general rather than limiting to accessLog

[wtzhang23]

Intended specifically for only using the CEL access log filter. I know it is possible to compose multiple access log filters together but I'd like to use CEL by itself for convenience sake. Especially should the expression change frequently.

I agree random is one of those features that would be useful everywhere, but it might be a good idea to not set the expectation that functions should be available everywhere. I also think that there is all sorts of functions that people would like and it would make sense to provide them the extensibility to compile in their own helper functions. Though we could probably keep it simple for this issue.

[tyxia]

If you are adding a CEL convenience function, it seems that it should not only be tied with access log, even if it is not available everywhere Limiting the scope to CEL SGTM

[tyxia]

cced @kyessenov as well

[wtzhang23]

Will try to add it everywhere I can once I open a PR

[wtzhang23]

Seems like x-request-id is already present so it may be better to simply use that? As in, in addition to the request.id attribute, we can add another attribute that converts it into a uint that can be used for randomness (e.g. use the request id extension's getInteger function)?

Anyways, I'm almost done with the function; just need to write unit tests.