Allow to implement caching in instrumentation

[pranas]

I know this change breaks your API but it gives more power to instrument_request method. You can use it to implement caching for example.

Please consider this change.

[Sutto]

I'd honestly rather introduce a method to do this, versus overriding the instrumentation to do so. That way we can maintain backwards compatibility but more importantly we're not using the instrumentation hooks to implement something that isn't instrumentation-related.

That said, I do like the change since it does give a little bit more power in instrumentation (namely, instrumenting on the return data as well)

[pranas]

I totally agree with you. Dedicated hook for caching purposes would be great.

I now think that this change is not that good after all. Besides that it breaks backwards compatibility, it lets people abuse instrument_request method.

What do you mean by "instrumenting on the return data"?