proxyd: Record latency of redis cache operations (#2442)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/late-swans-drum.md

+---
+'@eth-optimism/proxyd': patch
+---
+
+proxyd: Record redis cache operation latency

go/proxyd/cache.go

@@ -59,7 +59,10 @@ func newRedisCache(url string) (*redisCache, error) {
 }
 
 func (c *redisCache) Get(ctx context.Context, key string) (string, error) {
+	start := time.Now()
 	val, err := c.rdb.Get(ctx, key).Result()
+	redisCacheDurationSumm.WithLabelValues("GET").Observe(float64(time.Since(start).Milliseconds()))
+
 	if err == redis.Nil {
 		return "", nil
 	} else if err != nil {
@@ -70,7 +73,10 @@ func (c *redisCache) Get(ctx context.Context, key string) (string, error) {
 }
 
 func (c *redisCache) Put(ctx context.Context, key string, value string) error {
+	start := time.Now()
 	err := c.rdb.SetEX(ctx, key, value, redisTTL).Err()
+	redisCacheDurationSumm.WithLabelValues("SETEX").Observe(float64(time.Since(start).Milliseconds()))
+
 	if err != nil {
 		RecordRedisError("CacheSet")
 	}

go/proxyd/metrics.go

@@ -22,6 +22,7 @@ const (
 )
 
 var PayloadSizeBuckets = []float64{10, 50, 100, 500, 1000, 5000, 10000, 100000, 1000000}
+var MillisecondDurationBuckets = []float64{1, 10, 50, 100, 500, 1000, 5000, 10000, 100000}
 
 var (
 	rpcRequestsTotal = promauto.NewCounter(prometheus.CounterOpts{
@@ -198,6 +199,13 @@ var (
 		"gas price too low",
 		"invalid parameters",
 	}
+
+	redisCacheDurationSumm = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: MetricsNamespace,
+		Name:      "redis_cache_duration_milliseconds",
+		Help:      "Histogram of Redis command durations, in milliseconds.",
+		Buckets:   MillisecondDurationBuckets,
+	}, []string{"command"})
 )
 
 func RecordRedisError(source string) {