Round function should ignore native histograms

As per the documentation, native histograms are skipped. This is in line with other simpleFunc's.

Signed-off-by: Joshua Hesketh <josh@nitrotech.org>

promql/functions.go

@@ -551,6 +551,10 @@ func funcRound(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper
 	toNearestInverse := 1.0 / toNearest
 
 	for _, el := range vec {
+		if el.H != nil {
+			// Process only float samples.
+			continue
+		}
 		f := math.Floor(el.F*toNearestInverse+0.5) / toNearestInverse
 		enh.Out = append(enh.Out, Sample{
 			Metric:   el.Metric,

promql/promqltest/testdata/functions.test

@@ -1258,3 +1258,12 @@ load 1m
 # We expect the value to be 0 for t=0s to t=59s (inclusive), then 60 for t=60s and t=61s.
 eval range from 0 to 61s step 1s timestamp(metric)
   {} 0x59 60 60
+
+clear
+
+# Check round with mixed data types
+load 1m
+	mixed_metric {{schema:0 sum:5 count:4 buckets:[1 2 1]}} 1 2 3 {{schema:0 sum:5 count:4 buckets:[1 2 1]}} {{schema:0 sum:8 count:6 buckets:[1 4 1]}}
+
+eval range from 0 to 5m step 1m round(mixed_metric)
+	{} _ 1 2 3