fix: calculate conservative SLO percentiles

server/observability/slo.go

@@ -1,6 +1,7 @@
 package observability
 
 import (
+	"math"
 	"sort"
 	"time"
 )
@@ -52,7 +53,13 @@ func percentile(values []time.Duration, p float64) time.Duration {
 	}
 	ordered := append([]time.Duration(nil), values...)
 	sort.Slice(ordered, func(i, j int) bool { return ordered[i] < ordered[j] })
-	index := int(float64(len(ordered)-1) * p)
+	index := int(math.Ceil(p*float64(len(ordered)))) - 1
+	if index < 0 {
+		index = 0
+	}
+	if index >= len(ordered) {
+		index = len(ordered) - 1
+	}
 	return ordered[index]
 }