Support NaN values in metadata

A value of `float('nan')` gives this error:
```
  File "/opt/homebrew/Cellar/python@3.10/3.10.8/Frameworks/Python.framework/Versions/3.10/lib/python3.10/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
ValueError: Out of range float values are not JSON compliant
```