Add ip restriction for the prometheus_client output

[dupondje]

Sometimes you want to protect the exported /metrics path from the prometheus_client.
It was already possible to secure it with a password.

This patch adds the option to allow only defined CIDR ranges to access the /metrics.

[danielnelson]

@dupondje The code here is very good, but I feel like this addition won't be in very wide demand, and I don't want to support too many connection limitations as they are endless. Why not just set this up in your system firewall instead?

[dupondje]

@danielnelson: You could be correct that this isn't very wide demanded.

But this can be used in cases where firewalling isn't possible for example. Or when you want to have some additional protection mechanism.

Also I don't think there are endless ways to limit connections (that are supported by Prometheus itself).
Prometheus can only send credentials to the endpoint (this is already implemented).
Protecting via IP can be done without Prometheus needing to know this limitation.

Next to that, I think the patch is rather straightforward and small. So it doesn't add a lot of complexity to the code :)
And if you don't enable it, behaviour is unchanged.

Anyway, your call if you merge it or not!

[glinton]

Handle the error so the next line doesn't panic if user passes bad configuration

[dupondje]

Adjusted the code like requested.
Also put everything inside a if len(p.IPRange) > 0 {, cause we don't need to parse the RemoteIP if we don't set a ip_range restriction ofcourse. Saves some cycles :)