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>

configuration/configuration_test.go

@@ -84,7 +84,11 @@ var configStruct = Configuration{
 		} `yaml:"tls,omitempty"`
 		Headers http.Header `yaml:"headers,omitempty"`
 		Debug   struct {
-			Addr string `yaml:"addr,omitempty"`
+			Addr       string `yaml:"addr,omitempty"`
+			Prometheus struct {
+				Enabled bool   `yaml:"enabled,omitempty"`
+				Path    string `yaml:"path,omitempty"`
+			} `yaml:"prometheus,omitempty"`
 		} `yaml:"debug,omitempty"`
 		HTTP2 struct {
 			Disabled bool `yaml:"disabled,omitempty"`