trace2: use 'fmt' for the formatted message event field

In a message that is formatted/parameterised, we incorrectly use the
field name of 'format' rather than 'fmt'. Fix this.

Signed-off-by: Matthew John Cheetham <[email protected]>

Author: mjcheetham

src/shared/Core.Tests/Trace2MessageTests.cs

@@ -54,7 +54,7 @@ public void Event_Message_Without_Snake_Case_ToJson_Creates_Expected_Json()
             ParameterizedMessage = "baz"
         };
 
-        var expected = "{\"event\":\"error\",\"sid\":\"123\",\"thread\":\"main\",\"time\":\"0001-01-01T00:00:00+00:00\",\"file\":\"foo.cs\",\"line\":1,\"depth\":1,\"msg\":\"bar\",\"format\":\"baz\"}";
+        var expected = "{\"event\":\"error\",\"sid\":\"123\",\"thread\":\"main\",\"time\":\"0001-01-01T00:00:00+00:00\",\"file\":\"foo.cs\",\"line\":1,\"depth\":1,\"msg\":\"bar\",\"fmt\":\"baz\"}";
         var actual = errorMessage.ToJson();
 
         Assert.Equal(expected, actual);

src/shared/Core/Trace2Message.cs

@@ -409,7 +409,7 @@ public class ErrorMessage : Trace2Message
     [JsonPropertyOrder(8)]
     public string Message { get; set; }
 
-    [JsonPropertyName("format")]
+    [JsonPropertyName("fmt")]
     [JsonPropertyOrder(9)]
     public string ParameterizedMessage { get; set; }