Use CopyTo when resetting histogram in stats iterator

The histogram stats iterator does not fully clear the histogram object
and is not resilient to new fields being added to the histogram type.

To resolve the issue, the commit uses the CopyTo methods which should
be future proof to new fields being added.

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

promql/histogram_stats_iterator.go

@@ -62,9 +62,13 @@ func (f *histogramStatsIterator) AtHistogram(h *histogram.Histogram) (int64, *hi
 		return t, h
 	}
 
-	h.CounterResetHint = f.getResetHint(f.currentH)
-	h.Count = f.currentH.Count
-	h.Sum = f.currentH.Sum
+	returnValue := histogram.Histogram{
+		CounterResetHint: f.getResetHint(f.currentH),
+		Count:            f.currentH.Count,
+		Sum:              f.currentH.Sum,
+	}
+	returnValue.CopyTo(h)
+
 	f.setLastH(f.currentH)
 	return t, h
 }
@@ -89,9 +93,13 @@ func (f *histogramStatsIterator) AtFloatHistogram(fh *histogram.FloatHistogram)
 		return t, fh
 	}
 
-	fh.CounterResetHint = f.getFloatResetHint(f.currentFH.CounterResetHint)
-	fh.Count = f.currentFH.Count
-	fh.Sum = f.currentFH.Sum
+	returnValue := histogram.FloatHistogram{
+		CounterResetHint: f.getFloatResetHint(f.currentFH.CounterResetHint),
+		Count:            f.currentFH.Count,
+		Sum:              f.currentFH.Sum,
+	}
+	returnValue.CopyTo(fh)
+
 	f.setLastFH(f.currentFH)
 	return t, fh
 }