Prometheus metrics

Nobody has claimed this yet.

Hi!

Is there any publicly available code for easily exposing groupcache metrics for Prometheus?

I am sketching up the package below, but maybe I am reinventing the wheel here?!

Please advise!

```golang
// Usage example
{
metricsRoute := "/metrics"
metricsPort := ":3000"

log.Printf("starting metrics server at: %s %s", metricsPort, metricsRoute)

mailgun := mailgun.New(cache)
labels := map[string]string{
"app": appName,
}
namespace := ""
collector := groupcache_exporter.NewExporter(namespace, labels, mailgun)

prometheus.MustRegister(collector)

go func() {
http.Handle(metricsRoute, promhttp.Handler())
log.Fatal(http.ListenAndServe(metricsPort, nil))
}()
}
```

Full details: https://github.com/udhos/groupcache_exporter

Start by reviewing the linked groupcache_exporter package and its usage example to determine whether this repository should expose Prometheus metrics itself or point users to an external exporter. Done would require a decided integration scope and documented metric exposure approach; the issue names no repository files or tests to run.

Written by the indexing model from the issue text.