Make registry optional for MultiProcessCollector

[marksteward]

@brian-brazil this is a trivial first PR to make it possible to make it easier to add more collectors during a multiproc scrape. This is so the pattern eventually can be:

    registry = CollectorRegistry()
    registry.register(MultiProcessCollector())
    registry.register(PlatformCollector())
    # add your own CustomCollectors here

instead of having to explicitly pass None to each Collector.

I would also have included the change for PlatformCollector, but I'm concerned that might break existing code. Should it be accompanied by deprecation warnings, or is it not worth doing?

[brian-brazil]

I would also have included the change for PlatformCollector, but I'm concerned that might break existing code.

What is there to change?

[brian-brazil]

It's usually expected that you will pass the registry to a collector, rather than having to register it yourself. I'm not sure what you are proposing is actually saving code.

[marksteward]

What is there to change?

To make PlatformCollector default to not registering itself with the default registry, except for the default instance. So:

class PlatformCollector(object):
...
    def __init__(self, registry=None, platform=None):
...
PLATFORM_COLLECTOR = PlatformCollector(core.REGISTRY)

This is more explicit and means someone creating their own PlatformCollector instance doesn't end up with one already registered to the default registry by accident.

[marksteward]

It's usually expected that you will pass the registry to a collector, rather than having to register it yourself. I'm not sure what you are proposing is actually saving code.

This wasn't clear. The README has:

REGISTRY.register(CustomCollector())

but then:

    multiprocess.MultiProcessCollector(registry)

There's no mention of explicitly passing None to avoid the automatic registration.

When I first looked at this, I assumed MultiProcessCollector was doing some sort of quirky transformation on the registry, rather than just registering itself.

My hope is to make multiproc more self-explanatory and fit better with patterns used in Django, Flask, etc.

I'm happy to change the README instead if it should go the other way.

[brian-brazil]

So collectors are meant to auto-register with the default registry by default. The MultiProcessCollector in unusual in that you never use it with the default registry, and use a custom registry instead.

[marksteward]

I'll raise a separate PR to improve the README.