restore handling of the original level value

Author: KodrAus

src/SeqCli/Ingestion/JsonLogEventReader.cs

@@ -68,6 +68,18 @@ public static LogEvent ReadFromJObject(JObject jObject)
         if (!jObject.TryGetValue("@t", out _))
             jObject.Add("@t", new JValue(DateTime.UtcNow.ToString("O")));
 
+        if (jObject.TryGetValue("@l", out var levelToken))
+        {
+            var originalLevel = levelToken.Value<string>()!;
+            jObject.Remove("@l");
+
+            var serilogLevel = LevelMapping.ToSerilogLevel(originalLevel);
+            if (serilogLevel != LogEventLevel.Information)
+                jObject.Add("@l", new JValue(serilogLevel.ToString()));
+            
+            jObject.Add("SeqCliOriginalLevel", originalLevel);
+        }
+
         return LogEventReader.ReadFromJObject(jObject);
     }
 }

src/SeqCli/Output/OutputFormatter.cs

@@ -6,6 +6,6 @@ namespace SeqCli.Output;
 static class OutputFormatter
 {
     internal static readonly ITextFormatter Json = new ExpressionTemplate(
-        "{ {@t, @mt, @l: if @l = 'Information' then undefined() else @l, @x, @sp, @tr, @ps, @st, ..rest()} }\n"
+        "{ {@t, @mt, @l: coalesce(SeqCliOriginalLevel, if @l = 'Information' then undefined() else @l), @x, @sp, @tr, @ps, @st, ..rest()} }\n"
     );
 }

src/SeqCli/Syntax/SeqSyntax.cs

@@ -1,11 +1,28 @@
-using Seq.Syntax.Expressions;
+using System.Diagnostics.CodeAnalysis;
+using Seq.Syntax.Expressions;
 
 namespace SeqCli.Syntax;
 
 static class SeqSyntax
 {
     public static CompiledExpression CompileExpression(string expression)
     {
-        return SerilogExpression.Compile(expression);
+        return SerilogExpression.Compile(expression, nameResolver: new SeqCliNameResolver());
+    }
+}
+
+class SeqCliNameResolver: NameResolver
+{
+    public override bool TryResolveBuiltInPropertyName(string alias, [MaybeNullWhen(false)] out string target)
+    {
+        switch (alias)
+        {
+            case "@l":
+                target = "coalesce(SeqCliOriginalLevel, @l)";
+                return true;
+            default:
+                target = null;
+                return false;
+        }
     }
 }