fix(types): Registry.getSingleMetric may return undefined

[martin-helmich]

This PR fixes a small error in the TypeScript typings.

The implementation does a simple lookup on an object type:

	getSingleMetric(name) {
		return this._metrics[name];
	}

When supplied with a non-existent name, this will simply return undefined -- which is not reflected in the typings.

[SimenB]

thanks!