Allow for a custom MetricsCollector class to be loaded at runtime

[nipper]

Description

This adds in the ability to load a custom subclass of MetricsCollector at startup. This is so that if you want to have a private implementation of MetricsCollector you can.

It does this by adding the value of -1 to MetricsCollectors which represents Custom and an optional metrics_custom_import that is an optional string in the form of module.sub_module.ClassName. When both are provided it imports the class and returns an instance of it.

Motivation and Context

I need to have a private custom metrics collector that I'd like to use and this provides a backwards compatible way to do it.

Have you tested this? If so, how?

I've included a new unit test: test_custom_metrics_collector in test/scheduler_test.py

Since checks are blocked w/o maintainer approval I ran them here: https://github.com/nipper/luigi/pull/1/checks

[dlstadther]

Nice and clever! Sorry for such delay in review and merge

[nipper]

Thanks! No worries!