Add xray.AWSSession to install handlers on session

An application has to call xray.AWS for each AWS client it constructs.
This creates opportunities for blind spots if someone forgets to
configure a new client.

The xray.AWSSession installs the same handlers at the Session level.
Clients inherit handlers from the session they're created with. As long
as the application systematically reuses the same session to create
clients, it only needs to install X-Ray handlers once.

xray/aws.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws/client"
 	"github.com/aws/aws-sdk-go/aws/request"
+	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-xray-sdk-go/internal/logger"
 	"github.com/aws/aws-xray-sdk-go/resources"
 )
@@ -150,33 +151,44 @@ var xRayAfterRetryHandler = request.NamedHandler{
 	},
 }
 
-func pushHandlers(c *client.Client) {
-	c.Handlers.Validate.PushFrontNamed(xRayBeforeValidateHandler)
-	c.Handlers.Build.PushBackNamed(xRayAfterBuildHandler)
-	c.Handlers.Sign.PushFrontNamed(xRayBeforeSignHandler)
-	c.Handlers.Send.PushBackNamed(xRayAfterSendHandler)
-	c.Handlers.Unmarshal.PushFrontNamed(xRayBeforeUnmarshalHandler)
-	c.Handlers.Unmarshal.PushBackNamed(xRayAfterUnmarshalHandler)
-	c.Handlers.Retry.PushFrontNamed(xRayBeforeRetryHandler)
-	c.Handlers.AfterRetry.PushBackNamed(xRayAfterRetryHandler)
+func pushHandlers(handlers *request.Handlers, completionWhitelistFilename string) {
+	handlers.Validate.PushFrontNamed(xRayBeforeValidateHandler)
+	handlers.Build.PushBackNamed(xRayAfterBuildHandler)
+	handlers.Sign.PushFrontNamed(xRayBeforeSignHandler)
+	handlers.Send.PushBackNamed(xRayAfterSendHandler)
+	handlers.Unmarshal.PushFrontNamed(xRayBeforeUnmarshalHandler)
+	handlers.Unmarshal.PushBackNamed(xRayAfterUnmarshalHandler)
+	handlers.Retry.PushFrontNamed(xRayBeforeRetryHandler)
+	handlers.AfterRetry.PushBackNamed(xRayAfterRetryHandler)
+	handlers.Complete.PushFrontNamed(xrayCompleteHandler(completionWhitelistFilename))
 }
 
 // AWS adds X-Ray tracing to an AWS client.
 func AWS(c *client.Client) {
 	if c == nil {
 		panic("Please initialize the provided AWS client before passing to the AWS() method.")
 	}
-	pushHandlers(c)
-	c.Handlers.Complete.PushFrontNamed(xrayCompleteHandler(""))
+	pushHandlers(&c.Handlers, "")
 }
 
 // AWSWithWhitelist allows a custom parameter whitelist JSON file to be defined.
 func AWSWithWhitelist(c *client.Client, filename string) {
 	if c == nil {
 		panic("Please initialize the provided AWS client before passing to the AWSWithWhitelist() method.")
 	}
-	pushHandlers(c)
-	c.Handlers.Complete.PushFrontNamed(xrayCompleteHandler(filename))
+	pushHandlers(&c.Handlers, filename)
+}
+
+// AWSSession adds X-Ray tracing to an AWS session. Clients created under this
+// session will inherit X-Ray tracing.
+func AWSSession(s *session.Session) {
+	pushHandlers(&s.Handlers, "")
+}
+
+// AWSSessionWithWhitelist allows a custom parameter whitelist JSON file to be
+// defined.
+func AWSSessionWithWhitelist(s *session.Session, filename string) {
+	pushHandlers(&s.Handlers, filename)
 }
 
 func xrayCompleteHandler(filename string) request.NamedHandler {