docs: add API reference for CollectorRegistry and custom collector classes

[k1chik]

Closes #1163

Follows the same pattern as #1021 and #1162.

collector/custom.md: Collector protocol section (collect/describe), value vs labels mutual exclusivity note with correct per-type parameter names, full constructor and add_metric tables for GaugeMetricFamily, CounterMetricFamily, SummaryMetricFamily, HistogramMetricFamily, and InfoMetricFamily, plus a runnable real-world example.

collector/_index.md: constructor parameter tables for ProcessCollector, PlatformCollector, and GCCollector, with exported metrics listed for each.

registry/_index.md (new page): CollectorRegistry constructor and all public methods (register, unregister, collect, restricted_registry, get_sample_value, set_target_info, get_target_info), the global REGISTRY instance, and examples for isolated registry usage and registry=None.

All code examples verified by running them in Python.

cc @csmarchbanks

[csmarchbanks]

Thanks!

[calestyo]

Hey.

Thanks for improving on the documentation... that really helps a lot, compared to few years ago when I had to basically fiddle out most of these things from the code :-)

But one thing:

Why the yields in the collect()? I don’t quite get that.

Shouldn't collect() return sequences of *MetricFamily objects? You now return only a single one, and there even alternating ones like in:
a708480#diff-eadfec94068584be6fec5cce354e065ead34a754bc84ead564adf9e8c497ab10R237-R243

Or maybe you mean this as two independent examples?
What makes it confusing though is (which is however not from your commit):

client_python/docs/content/collector/custom.md

Lines 16 to 21 in 482656c

	def collect(self):
	yield GaugeMetricFamily('my_gauge', 'Help text', value=7)
	c = CounterMetricFamily('my_counter_total', 'Help text', labels=['foo'])
	c.add_metric(['bar'], 1.7)
	c.add_metric(['baz'], 3.8)
	yield c

There collect() alternates between returning (well yielding) a Gauge and Counter (also not as sequences)?

I generally don't quite understand why one would yield here at all.

Isn't the reason for custom collectors to start every time fresh (and get rid of all stale metrics, labels, etc.)?
With yield wouldn't you keep all that?

Thanks,
Chris.

[calestyo]

Shouldn't do stuff late night... yield of course makes it an iterator o.O

Still I wonder, is there any advantage over using that in the current form rather than return?

I mean is Python' GC smart enough that it can already clean the objects that have already been yielded and thus give an advantage by reducing e.g. memory use for very large metric sets?

In:

        yield GaugeMetricFamily('my_gauge', 'Help text', value=7)
        c = CounterMetricFamily('my_counter_total', 'Help text', labels=['foo'])
        c.add_metric(['bar'], 1.7)
        c.add_metric(['baz'], 3.8)
        yield c

I could imagine that it may release the GaugeMetricFamily object early... but c? It still has a reference?

In any case it would be nice to have in the docs why generators are used (if it's memory advantages) and how to use them so that this actually works out. :-)