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.go

@@ -129,6 +129,11 @@ type Configuration struct {
 		Debug struct {
 			// Addr specifies the bind address for the debug server.
 			Addr string `yaml:"addr,omitempty"`
+			// Prometheus configures the Prometheus telemetry endpoint.
+			Prometheus struct {
+				Enabled bool   `yaml:"enabled,omitempty"`
+				Path    string `yaml:"path,omitempty"`
+			} `yaml:"prometheus,omitempty"`
 		} `yaml:"debug,omitempty"`
 
 		// HTTP2 configuration options