Add Prometheus Metrics

at the first iteration, only the following metrics are collected:

  - HTTP metrics of each API endpoint
  - cache counter for request/hit/miss
  - histogram of storage actions, including:
    GetContent, PutContent, Stat, List, Move, and Delete

Signed-off-by: tifayuki <tifayuki@gmail.com>

cmd/registry/config-dev.yml

@@ -31,7 +31,10 @@ storage:
 http:
     addr: :5000
     debug:
-        addr: localhost:5001
+        addr: :5001
+        prometheus:
+            enabled: true
+            path: /metrics
     headers:
         X-Content-Type-Options: [nosniff]
 redis: