Support float64 in enum processor

[tuannh99]

This adds float64 support to enum processor.

Required for all PRs:

• Associated README.md updated.
• Has appropriate unit tests.

[telegraf-tiger]

🤝 ✅ CLA has been signed. Thank you!

[srebhan]

@tuannh99 while the code looks good I wonder what your use case is. As far as I get it you want to replace a float value with something else?
However, float values are notoriously difficult to compare and your are trying to convert a dense quantity into a categorical value which is ill-defined anyway without quantization.
To cut a long story short, please outline your use-case. If you really need this I would expect at least an option float_precision and maybe additionally float_rounding which can be floor, ceil and round at least...

[srebhan]

@tuannh99 any news on this?

[tuannh99]

@srebhan I should have a commit to update this in a few.

[srebhan]

@tuannh99 can you please comment on my concern above that trying to map a raw floating point value is very fragile and explain what your use-case is

[ssoroka]

This is fine as is. In a future PR we may want to expose the FormatFloat() precision argument, to give users the ability to control how precisely the floats are matched.