message: subscriber: Add the closing } to the JSON logs
  ## Motivation
  JSON log lines are not correct JSON, the ending `}` is missing.
  ## Solution
  Ending the `SerializeMap` correctly.
diff: @@ -58,7 +58,8 @@ where
  serializer.serialize_entry("target", meta.target())?;
  }
  - serializer.serialize_entry("fields", &event.field_map())
  + serializer.serialize_entry("fields", &event.field_map())?;
  + serializer.end()
  };
  visit().map_err(|_| fmt::Error)?;
  @@ -264,7 +265,7 @@ mod test {
  let make_writer = || MockWriter::new(&BUF);
  let expected =
  - "{\"timestamp\":\"fake time\",\"level\":\"INFO\",\"span\":{\"name\":\"json_span\",\"fields\":\"{\\\"answer\\\":42,\\\"number\\\":3}\"},\"target\":\"tracing_subscriber::fmt::format::json::test\",\"fields\":{\"message\":\"some json test\"}\n";
  + "{\"timestamp\":\"fake time\",\"level\":\"INFO\",\"span\":{\"name\":\"json_span\",\"fields\":\"{\\\"answer\\\":42,\\\"number\\\":3}\"},\"target\":\"tracing_subscriber::fmt::format::json::test\",\"fields\":{\"message\":\"some json test\"}}\n";
  test_json(make_writer, expected, &BUF);
  }
  