Calculate correlation between two keys

[maital]

Hi,

I'm suggesting a feature to calculate Pearson Correlation Coefficient between two keys.
Two applications I could think of for this feature include:

• Gene Expression Correlation: for example, for constructing Gene co-expression networks
• System monitoring: for example, create alert when a negative correlation is detected between CPU usage and incoming requests

I hope you'll find it interesting.
Please see PR #23
Thanks 🌷