add LimitNOFILE to prometheus server unit

[sc0rp10]

As a Brian Brazil said nofile limit should be set to an large value, one billion as example.
I think that we should set this in service unit because of any other ways may not work unlike systemd.

[bastelfreak]

HI @sc0rp10, thanks for the PR. I don't think that such a high value is a good default. Could you make it configureable and optional?

[sc0rp10]

Hi, @bastelfreak I was guided by Brian's article that I linked earlier, what do you think about it?

[bastelfreak]

This value is way to high for small installations. So I think it should be an optional parameter.

[sc0rp10]

what default value are you prefer?

[alexjfisher]

I think the idea proposed is that you make it an Optional[Integer] parameter that defaults to undef. Then in the template you only write the line if the value was set to something.

[bastelfreak]

Ping @sc0rp10 :)

[dhoppe]

@bastelfreak I would like to close this pull request in favor of #298. Any objections?