Support for bias/constant in linear scaling of gauge values

[joe-macmillan]

Issue Summary:

When linearly scaling the values read from registers, certain sensors/devices may require adjustment for circuit bias. For instance, I have an Advantech device that needs converting register values using a linear function to obtain correct voltages.

The current version of the modbus-exporter only supports a factor for scaling. Therefore, I had to fork the project and add support for bias as well, which lets me scale my readings with the formula value = factor * reading - bias.

Proposed Solution:

I've created a fork of the modbus-exporter that incorporates support for the bias parameter. I would be happy to share my fork here for consideration. This addition could provide users with more flexibility in scaling register values.

Future Development:

While my current fork is allowing for linear scaling, I also may need support for polynomial conversion of values in the future.

Your feedback and thoughts on this feature would be greatly appreciated.