Add redis pool to registry webapp

Redis has been integrated with the web application for use with various
services. The configuraiton exposes connection details, timeouts and pool
parameters. Documentation has been updated accordingly.

A few convenience methods have been added to the context package to get loggers
with certain fields, exposing some missing functionality from logrus.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

configuration/configuration.go

@@ -91,6 +91,36 @@ type Configuration struct {
 	// Notifications specifies configuration about various endpoint to which
 	// registry events are dispatched.
 	Notifications Notifications `yaml:"notifications,omitempty"`
+
+	// Redis configures the redis pool available to the registry webapp.
+	Redis struct {
+		// Addr specifies the the redis instance available to the application.
+		Addr string `yaml:"addr,omitempty"`
+
+		// Password string to use when making a connection.
+		Password string `yaml:"password,omitempty"`
+
+		// DB specifies the database to connect to on the redis instance.
+		DB int `yaml:"db,omitempty"`
+
+		DialTimeout  time.Duration `yaml:"dialtimeout,omitempty"`  // timeout for connect
+		ReadTimeout  time.Duration `yaml:"readtimeout,omitempty"`  // timeout for reads of data
+		WriteTimeout time.Duration `yaml:"writetimeout,omitempty"` // timeout for writes of data
+
+		// Pool configures the behavior of the redis connection pool.
+		Pool struct {
+			// MaxIdle sets the maximum number of idle connections.
+			MaxIdle int `yaml:"maxidle,omitempty"`
+
+			// MaxActive sets the maximum number of connections that should be
+			// opened before blocking a connection request.
+			MaxActive int `yaml:"maxactive,omitempty"`
+
+			// IdleTimeout sets the amount time to wait before closing
+			// inactive connections.
+			IdleTimeout time.Duration `yaml:"idletimeout,omitempty"`
+		} `yaml:"pool,omitempty"`
+	} `yaml:"redis,omitempty"`
 }
 
 // v0_1Configuration is a Version 0.1 Configuration struct