Use the timestamp of the original log event

Fixes the issue that log messages of one batch all get the time of preparing the batch as timestamps. Now the timestamp that comes with the Serilog log event is used.

src/Serilog.Sinks.Loki/LokiBatchFormatter.cs

@@ -54,9 +54,7 @@ public void Format(IEnumerable<LogEvent> logEvents, ITextFormatter formatter, Te
                     // We also remove backslashes and replace with forward slashes, Loki doesn't like those either
                     stream.Labels.Add(new LokiLabel(property.Key, property.Value.ToString().Replace("\"", "").Replace("\r\n", "\n").Replace("\\", "/")));
 
-                var localTime = DateTime.Now;
-                var localTimeAndOffset = new DateTimeOffset(localTime, TimeZoneInfo.Local.GetUtcOffset(localTime));
-                var time = localTimeAndOffset.ToString("o");
+                var time = logEvent.Timestamp.ToString("o");
 
                 var sb = new StringBuilder();
                 sb.AppendLine(logEvent.RenderMessage());