Clarify type and meaning of stacks_* metrics

The stacks_failing metric is created as a GaugeVec in the Go code, which represents a set of time series distinguished by labels (in this case, "namespace" and "name"). But each of these time series are of type `gauge`, so the documentation is misleading in referring to them as `gaugevec` (which is not a kind of metric).

I've simplified the verbiage a little, in passing.

Addresses #399.

docs/metrics.md

@@ -20,8 +20,8 @@ Once the above are created, Prometheus will update its target scraping rules to
 
 The current implementation explicitly emits the following metrics:
 
-1. `stacks_active` - `gauge` that tracks the number of currently registered stacks managed by the system
-2. `stacks_failing` - `gaugevec` that provides information about stacks currently failing (`stack.status.lastUpdate.state` is `failed`)
+1. `stacks_active` - a `gauge` time series that reports the number of currently registered stacks managed by the system
+2. `stacks_failing` - a set of `gauge` time series, labelled by namespace, that gives the number of stacks currently failing (`stack.status.lastUpdate.state` is `failed`)
 
 In addition, we find tracking the following metrics emitted by the controller-runtime would be useful to track: