handle original level in plain text logs

Author: KodrAus

src/SeqCli/PlainText/LogEvents/LogEventBuilder.cs

@@ -38,6 +38,7 @@ public static LogEvent FromProperties(IDictionary<string, object?> properties, s
         var props = GetLogEventProperties(properties, remainder, level);
 
         var fallbackMappedLevel = level != null ? LevelMapping.ToSerilogLevel(level) : LogEventLevel.Information;
+        properties["SeqCliOriginalLevel"] = level;
 
         return new LogEvent(
             timestamp,

src/SeqCli/PlainText/PlainTextLogEventReader.cs

@@ -35,6 +35,7 @@ public async Task<ReadResult> TryReadAsync()
             throw new InvalidDataException($"A line arrived late or could not be parsed: `{frame.Value.Trim()}`.");
 
         var (properties, remainder) = _nameValueExtractor.ExtractValues(frame.Value);
+
         var evt = LogEventBuilder.FromProperties(properties, remainder);
         return new ReadResult(evt, frame.IsAtEnd);
     }