Integrate sprig template function library

[hamiltont]

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The templates directory is nice, but given the range of data that could pass through grok_exporter perhaps something like http://masterminds.github.io/sprig/ should be incorporated. For example, I'm trying to use the output of subtract to slice, but unfortunately it outputs float64 while slice requires an int type. Having a larger body of helper functions would allow handling more edge case situations

[fstab]

I agree, more functions would be good. I'll have a look if it's possible to implement the Sprig functions in grok_exporter.