Added ability to specify a directory of config files

[omnistat]

After using this plugin for a little while, it started to become burdensome to navigate one huge configuration file for every metric of multiple modbus devices. I read through some issues and saw others mention this as well.

I decided to add basic functionality to specify either a single configuration file or a directory of configuration files in the format of modbus.yml. This will load all configurations as if they were in one file, but allows users to seperate them out for readability.

The folder name must end with .d to distinguish between a file or folder. For code simplicity, all files within the directory will attempt to be loaded, including non-yaml files, and error accordingly.

Example:

modbus_configs.d
├── module1.yml
└── module2.yml

./modbus_exporter --config="modbus_configs.d"

Reflecting this change, the --config.file parameter has been changed to --config to be more general.

[SuperQ]

This is not the way to handle this. Take a look how the multiple files/glob handling was done in the snmp_exporter.

prometheus/snmp_exporter#970

[omnistat]

Thanks for the correction. I'll work on implementing this method instead. I wasn't aware this existed with the snmp exporter but I can see the rationale.

[omnistat]

How is this? It was quite simple to implement and functions very similarly to snmp_exporter. I'm not sure if the change warrants an additional test case.

[SuperQ]

Nice, thanks!

[bastischubert]

@omnistat could you please update the readme.md that has a conflict, so i can merge this pr ;)

[omnistat]

@bastischubert Done :)