Fix scrape timestamps to reduce sample time jitter.

We're currently timestamping samples with the time at the end of a scrape
iteration. It makes more sense to use a timestamp from the beginning of the
scrape for two reasons:

a) this time is more relevant to the scraped values than the time at the
   end of the HTTP-GET + JSON decoding work.
b) it reduces sample timestamp jitter if we measure at the beginning, and
   not at the completion of a scrape.