promqltest: Add test to demonstrate sub-query bug with HistogramStatsIterator

To fix this, we need to make sure that HistogramStatsIterator is not
used with a sub-query in the path.

Signed-off-by: beorn7 <beorn@grafana.com>

promql/promqltest/testdata/native_histograms.test

@@ -1608,6 +1608,24 @@ eval instant at 1m histogram_quantile(0.5, myHistogram2)
 eval instant at 1m histogram_quantile(0.5, mixedHistogram)
 	expect warn msg: PromQL warning: vector contains a mix of classic and native histograms for metric name "mixedHistogram"
 
+clear
+
+# A counter reset only in a bucket. Sub-queries still need to detect
+# it via explicit counter reset detection. This test also runs it with
+# histogram_count in the expression to make sure that the
+# HistogramStatsIterator is not used. (The latter fails to correctly
+# do the counter resets because Seek is used with sub-queries. And the
+# explicit counter reset detection done with sub-queries cannot access
+# the buckets anymore, if HistogramStatsIterator is used.)
+load 1m
+     h{} {{schema:0 count:1 sum:10 buckets:[1]}}+{{}}x20 {{schema:0 count:1 sum:10 buckets:[0 1]}}+{{}}x20
+
+# Both evals below should yield the same value for the count.
+eval instant at 41m histogram_count(increase(h[40m:9m]))
+    {} 1.4814814814814814
+
+eval instant at 41m increase(h[40m:9m])
+    {} {{count:1.4814814814814814 sum:14.814814814814813 counter_reset_hint:gauge offset:1 buckets:[1.4814814814814814]}}
 
 clear