Documentation improvements

[elliotdavies]

The docs for this library are really good! A couple of small things I noticed:

generic-prometheus/src/GenericPrometheus.hs

Line 230 in dbe006f

-- Prom.withMetricIO _msGauge (Prom.setGauge 3.0)

I think this needs a flip like so: Prom.withMetricIO _msGauge (flip Prom.setGauge 3.0). That may also be true for the other examples in that section.

generic-prometheus/src/GenericPrometheus.hs

Lines 136 to 137 in dbe006f

	-- You can add support for Prometheus metrics by @deriving@ an instance of
	-- 'MonadPrometheus' @via@ this module's 'PrometheusT' type:

If I'm inside App and use Prom.withMetric with a function like Prom.setGauge then I'm told there's no instance for Prom.MonadMonitor App. (Prom.withMetricIO works fine.) Perhaps I made a mistake, but if not it would be worth documenting that the instance is needed.