Support matching JSON body with CEL expressions

[juho9000]

This PR implements parsing a JSON response from the body and checking the result using a CEL query. Looking for feedback on this.

Sample usage:

modules:
  http_2xx:
    prober: http
    http:
      fail_if_body_json_not_matches_cel: "body.foo.bar == 'baz'"

If response is {"foo": { "bar": "qux" } } the probe fails. fail_if_body_json_matches_cel is also implemented and should work as expected.

[juho9000]

@roidelapluie @mem Ping :)

[mem]

A couple of minor comments.

[mem]

I'm thinking this should be added conditionally, if the configuration asks for validation using CEL expressions.

[juho9000]

I was following the same logic that is applied to regex checks, following your logic that should also be conditionally added, no?

[mem]

Thank you for this, this seems useful!

Question, I have no experience with CEL... is it possible to use it to match HTML, too? I see in the code that Eval takes a map[string]any, but I couldn't spot how that any is handled.

The reason I'm asking is because I'm trying to figure out if it's possible to extend this to more inputs, or if it needs to be restricted to JSON.

[mem]

Question: shouldn't this be EvalContext instead? I'm not clear about the guarantees that the evaluation will terminate, and the fact that EvalContext exists suggests that there's a benefit from imposing a deadline.

[juho9000]

You're probably right, I've changed it to use ContextEval

[juho9000]

Thank you for this, this seems useful!

Question, I have no experience with CEL... is it possible to use it to match HTML, too? I see in the code that Eval takes a map[string]any, but I couldn't spot how that any is handled.

The reason I'm asking is because I'm trying to figure out if it's possible to extend this to more inputs, or if it needs to be restricted to JSON.

I atleast do not know how CEL could be used to match HTML in any sane way. You might want to check this out https://github.com/google/cel-spec/blob/master/doc/langdef.md#json-data-conversion

Thanks for your comments and suggestions so far. Really hoping we can get this merged. I think that having the ability to validate JSON responses would be an awesome addition to blackbox exporter. :)

[zetaab]

@mem kindly ping :) we are interested about this feature and would like to see it as well

[zetaab]

@mem @roidelapluie any comments to this PR?