[1.x] Customise server stats

[timacdonald]

fixes #322

Allows the server stat collection to be customised.

In a service provider it is now easy to customise the CPU and memory stat collection depending on your special operating system or unique infrastructure (docker, et al.).

Servers::detectCpuUsing(function (): int {
    // ...
});

Servers::detectMemoryUsing(fn () => [
    'total' => (int) /* ... */,
    'used' => (int) /* ... */,
]);

[jessarcher]

Dig this!

[justinkekeocha]

any documentation on how this can be used?