allow other plugins to register metrics

[terinjokes]

If there's interest from other developers, we could allow for external scripts to expose their own metrics (eg, a Twitch plugin could have metrics for responding to chat commands).

1. What APIs do these developers need? We'd probably want to expose the fundamental types and the default registry at the very least.
2. How should we handle OpenRCT2 plugin loading order? Other plugins may load before us. AFAICT, there's no hook for plugins loaded, so not a great way to signal to other plugins that we're now available. Pooling for our global could work, but not great.
3. Allow users to disable metrics from specific plugins. We might be able to do this as part of a configuration UI (and initially the configuration file)