lib/protoparser/common: do not read request body when parsing timestamp query arg

This was preventing from reading data via /api/v1/prometheus/import .

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/750

lib/protoparser/common/timestamp.go

@@ -10,7 +10,7 @@ import (
 //
 // It returns 0 if there is no `timestamp` query arg.
 func GetTimestamp(req *http.Request) (int64, error) {
-	ts := req.FormValue("timestamp")
+	ts := req.URL.Query().Get("timestamp")
 	if len(ts) == 0 {
 		return 0, nil
 	}