Include response body in sample

[blalor]

Someday I will live in a world where the HTTP status code of a service response will be sufficient for determining the proper status of that response. Still trying to find that place, however. It would be useful to return the body of the response as a property of the sample for further analysis outside of the core canary library.

Should be fairly straightforward; the body's already being read:

canary/pkg/sampler/sampler.go

Line 101 in 427ed9e

val := sample.ResponseHeaders.Get("Content-Length")

Support will probably be required for chunked encoding.