Fix last_over_time for native histograms

The last_over_time retains a histogram sample without making a copy.
This sample is now coming from the buffered iterator used for windowing functions,
and can be reused for reading subsequent samples as the iterator progresses.

I would propose copying the sample in the last_over_time function, similar to
how it is done for rate, sum_over_time and others.

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

promql/functions.go

@@ -603,7 +603,7 @@ func funcLastOverTime(vals []parser.Value, args parser.Expressions, enh *EvalNod
 	}
 	return append(enh.Out, Sample{
 		Metric: el.Metric,
-		H:      h.H,
+		H:      h.H.Copy(),
 	}), nil
 }
 

promql/testdata/native_histograms.test

@@ -224,3 +224,14 @@ eval instant at 5m histogram_fraction(0, 4, balanced_histogram)
 # the first populated bucket after the span of empty buckets.
 eval instant at 5m histogram_quantile(0.5, balanced_histogram)
 	{} 0.5
+
+# Add histogram to test sum(last_over_time) regression
+load 5m
+    incr_sum_histogram{number="1"} {{schema:0 sum:0 count:0 buckets:[1]}}+{{schema:0 sum:1 count:1 buckets:[1]}}x10
+    incr_sum_histogram{number="2"} {{schema:0 sum:0 count:0 buckets:[1]}}+{{schema:0 sum:2 count:1 buckets:[1]}}x10
+
+eval instant at 50m histogram_sum(sum(incr_sum_histogram))
+    {} 30
+
+eval instant at 50m histogram_sum(sum(last_over_time(incr_sum_histogram[5m])))
+    {} 30