promql: Use HistogramStatsIterator more often

The current code stops the walk after we have found the first relevant
function. However, in expressions with multiple legs, we will then use
the `HistogramStatsIterator` at most once. This change should make
sure we explore all legs.

The added tests make sure we are not using `HistogramStatsIterator`
where we shouldn't (but the opposite can only be seen in a benchmark
or with a more explicit test).

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

promql/engine.go

@@ -3920,7 +3920,7 @@ func detectHistogramStatsDecoding(expr parser.Expr) {
 				break pathLoop
 			}
 		}
-		return errors.New("stop")
+		return nil
 	})
 }
 

promql/promqltest/testdata/native_histograms.test

@@ -1660,3 +1660,20 @@ eval instant at 30s histogram_count(avg(reset))
 eval instant at 5m histogram_count(rate(reset{timing="late"}[5m]))
     expect no_warn
     {timing="late"} 0.0175
+
+clear
+
+# Test edge cases of HistogramStatsIterator detection.
+# We access the same series multiple times within the same expression,
+# once with and once without HistogramStatsIterator. The results here
+# at least prove that we do not use HistogramStatsIterator where we
+# should not.
+load 1m
+     histogram {{schema:0 count:10 sum:50 counter_reset_hint:gauge buckets:[1 2 3 4]}}
+
+eval instant at 1m histogram_count(histogram unless histogram_quantile(0.5, histogram) < 3)
+     {} 10
+
+eval instant at 1m histogram_quantile(0.5, histogram unless histogram_count(histogram) == 0)
+     {} 3.1748021039363987
+