Unable to print msgpack to stdout using out_lib.c

## Bug Report

**Describe the bug**
I am trying to use the [out_lib.c example](https://github.com/fluent/fluent-bit/blob/master/examples/out_lib/out_lib.c). The `printf` to stdout works fine when using the JSON format, however when I am trying the commented out code to print to stdout using the msgpack format, I am getting the following result:
`[my_stdout_msgpack]#`

**To Reproduce**
Use the [out_lib.c example](https://github.com/fluent/fluent-bit/blob/master/examples/out_lib/out_lib.c) example and replace the default output that uses JSON with the commented out msgpack format output.

**Additional context**
I am trying to embed Fluent Bit into my application using the provided C API. While I can read my logs in using the tail plugin, I can parse them fine and I can also print them to stdout in JSON format, I cannot print them using the msgpack format (I need this functionality for debugging purposes but also to understand how msgpack works, so that I can extract the required data - unless there is a better way to extract the parsed log values using their keys?).