Make a decision about examples from mock values and real values

[ArcturusZhang]

Since we have the ability of generating examples from real example values (reading from the json files examples in the spec repo), we need to decide how we deal with those operations without a real example.

When there is an examples-dir configured, for those operations that do not have an example file, should we continue to generate examples with mock values?

In the implementation in this PR, we do not. I think it might be misleading because the examples from mock values look exactly the same as the ones from real values - there is no distinguishable flags or anything to mark those from mock values. Therefore I think it is a bad idea of mixing them up.