meta openapi:example work at APIExpr level too

[antipopp]

As mentioned in my issue #3316 this meta doesn't work as the documentation says.

// - "swagger:example" DEPRECATED, use "openapi:example" instead
//
// - "openapi:example" specifies whether to generate random example. Defaults to
// true. Applicable to API (applies to all attributes) or individual attributes.
//
//	var _ = API("MyAPI", func() {
//	    Meta("openapi:example", "false")
//	})

I've looked into the code for a while and really couldn't find a better way than this. The problem is that examples are generated on the Attribute expression, and the meta will work only there. My solution checks for the API meta during the generation of the openapi schema and sets the random generator seed to nil, and then adding the check to the Example method that created the example.

I found the codebase kinda confusing and I won't be surprised if there's a better, cleaner way, but since nobody was looking into it I tried myself.

[raphael]

Great first stab! A few things that would make this a bit better:

1. It would suffice to change the generator just once in the New function of builder.go.
2. The metadata check could simply call MustGenerate(api.Meta) instead of testing inline
3. I would suggest setting the Randomizer of the ExampleGenerator to nil instead of the generator itself. The test to skip generation would be changed to if r.Randomizer == nil. We could document the behavior in expr/random.go (there are currently two possible randomizer types)

[antipopp]

Let me know for other suggestions, somehow I totally missed the New function while I was traversing the whole flow when looking into it :)

About the inline check, there is already a MustGenerate function that checks if the whole schema needs to be generated. I made a new function that checks only for the example meta instead of mixing the two things. Hopefully you meant it in this way.

[raphael]

Looks good - left one question!

[raphael]

Thank you!