Add instrumentation to the User cache

So we can monitor its effectiveness and validate design choices.

backend/cache/cache.go

@@ -66,7 +66,7 @@ func Wrap(b as.Backend, peers []string, tls *clientutil.TLSClientConfig) (*Cache
 	return &CacheBackend{
 		Backend: b,
 		Handler: h,
-		cache:   c,
+		cache:   instrument(c),
 	}, nil
 }
 

backend/cache/instrumentation.go

+package cachebackend
+
+import (
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// There's only going to be one cache per server process in production
+// so we can just use globals for instrumentation metrics.
+var (
+	cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "accountserver_cache_hits",
+		Help: "Cache hit counter",
+	})
+	cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "accountserver_cache_misses",
+		Help: "Cache miss counter",
+	})
+	cacheWrites = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "accountserver_cache_writes",
+		Help: "Cache write counter",
+	})
+	cacheDeletes = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "accountserver_cache_deletes",
+		Help: "Cache delete counter",
+	})
+)
+
+func init() {
+	prometheus.MustRegister(cacheHits)
+	prometheus.MustRegister(cacheMisses)
+	prometheus.MustRegister(cacheWrites)
+	prometheus.MustRegister(cacheDeletes)
+}
+
+type instrumentedCache struct {
+	internalCache
+}
+
+func instrument(c internalCache) *instrumentedCache {
+	return &instrumentedCache{c}
+}
+
+func (c *instrumentedCache) Get(key string) (interface{}, bool) {
+	res, ok := c.internalCache.Get(key)
+	if ok {
+		cacheHits.Inc()
+	} else {
+		cacheMisses.Inc()
+	}
+	return res, ok
+}
+
+func (c *instrumentedCache) Set(key string, value interface{}, d time.Duration) {
+	c.internalCache.Set(key, value, d)
+	cacheWrites.Inc()
+}
+
+func (c *instrumentedCache) Delete(username string) {
+	c.internalCache.Delete(username)
+	cacheDeletes.Inc()
+}