downloader: show correct rates after restart (#13324)

Set initial values `prevStats.BytesDownload` and
`prevStats.BytesCompleted` if they are zero, which can occur if the node
was restarted or launched for first time.
Fix for #13250 and
#13044

erigon-lib/downloader/downloader.go

@@ -1973,6 +1973,15 @@ func (d *Downloader) ReCalcStats(interval time.Duration) {
 	stats.BytesDownload = uint64(connStats.BytesReadData.Int64())
 	stats.BytesCompleted = uint64(connStats.BytesCompleted.Int64())
 
+	// if we have no previous stats, it means that node was restarted and need to set initial values
+	if prevStats.BytesDownload == 0 {
+		prevStats.BytesDownload = stats.BytesCompleted
+	}
+
+	if prevStats.BytesCompleted == 0 {
+		prevStats.BytesCompleted = stats.BytesCompleted
+	}
+
 	lastMetadataReady := stats.MetadataReady
 
 	stats.BytesTotal, stats.ConnectionsTotal, stats.MetadataReady = 0, 0, 0