proxyd/fix: error rate tolerance (#5606)

proxyd/backend.go

@@ -557,7 +557,11 @@ func (b *Backend) doForward(ctx context.Context, rpcReqs []*RPCReq, isBatch bool
 
 // IsHealthy checks if the backend is able to serve traffic, based on dynamic parameters
 func (b *Backend) IsHealthy() bool {
-	errorRate := b.networkErrorsSlidingWindow.Sum() / b.networkRequestsSlidingWindow.Sum()
+	errorRate := float64(0)
+	// avoid division-by-zero when the window is empty
+	if b.networkRequestsSlidingWindow.Sum() >= 10 {
+		errorRate = b.networkErrorsSlidingWindow.Sum() / b.networkRequestsSlidingWindow.Sum()
+	}
 	avgLatency := time.Duration(b.latencySlidingWindow.Avg())
 	if errorRate >= b.maxErrorRateThreshold {
 		return false