Return just observed sample for Histogram observeAndSample

The Histogram metric has a function `observeAndSample`, which lets you
add a new observation, and sample it at the same time.  Before this
commit, this `observeAndSample` function would return the _previous_
sample, _not_ the sample you just added.

This commit changes the behavior of `observeAndSample` so that it
returns the sample that was just added.  This matches the behavior
of `addAndSample` (from `Counter`), and `modifyAndSample` (from
`Gauge`), which both return the value of the sample that was just
added.

src/System/Metrics/Prometheus/Metric/Histogram.hs

@@ -50,7 +50,7 @@ new buckets = Histogram <$> newIORef empty
 observeAndSample :: Double -> Histogram -> IO HistogramSample
 observeAndSample x = flip atomicModifyIORef' update . unHistogram
   where
-    update histData = (hist' histData, histData)
+    update histData = (hist' histData, hist' histData)
     hist' histData =
         histData
             { histBuckets = updateBuckets x $ histBuckets histData