fix: use better value for threshold value in alert description

pkg/query-service/rules/prom_rule.go

@@ -593,13 +593,32 @@ func (r *PromRule) shouldAlert(series pql.Series) (pql.Sample, bool) {
 					break
 				}
 			}
+			// use min value from the series
+			if shouldAlert {
+				var minValue float64 = math.Inf(1)
+				for _, smpl := range series.Floats {
+					if smpl.F < minValue {
+						minValue = smpl.F
+					}
+				}
+				alertSmpl = pql.Sample{F: minValue, Metric: series.Metric}
+			}
 		} else if r.compareOp() == ValueIsBelow {
 			for _, smpl := range series.Floats {
 				if smpl.F >= r.targetVal() {
 					shouldAlert = false
 					break
 				}
 			}
+			if shouldAlert {
+				var maxValue float64 = math.Inf(-1)
+				for _, smpl := range series.Floats {
+					if smpl.F > maxValue {
+						maxValue = smpl.F
+					}
+				}
+				alertSmpl = pql.Sample{F: maxValue, Metric: series.Metric}
+			}
 		} else if r.compareOp() == ValueIsEq {
 			for _, smpl := range series.Floats {
 				if smpl.F != r.targetVal() {
@@ -614,6 +633,14 @@ func (r *PromRule) shouldAlert(series pql.Series) (pql.Sample, bool) {
 					break
 				}
 			}
+			if shouldAlert {
+				for _, smpl := range series.Floats {
+					if !math.IsInf(smpl.F, 0) && !math.IsNaN(smpl.F) {
+						alertSmpl = pql.Sample{F: smpl.F, Metric: series.Metric}
+						break
+					}
+				}
+			}
 		}
 	case OnAverage:
 		// If the average of all samples matches the condition, the rule is firing.