Log that the duplicate key error patch is applied

Otherwise the only way we have to check is to try to cause an E11000
error, which is time-consuming and false-negative-prone.

Log this at debug level - it should still show up in the Juju logging
before an agent switches to the configured model logging level.

session.go

@@ -41,6 +41,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/juju/loggo"
 	"gopkg.in/mgo.v2/bson"
 )
 
@@ -144,6 +145,9 @@ type Iter struct {
 var (
 	ErrNotFound = errors.New("not found")
 	ErrCursor   = errors.New("invalid cursor")
+
+	logPatchedOnce sync.Once
+	logger         = loggo.GetLogger("mgo")
 )
 
 const (
@@ -244,6 +248,9 @@ const (
 //     http://docs.mongodb.org/manual/reference/connection-string/
 //
 func Dial(url string) (*Session, error) {
+	logPatchedOnce.Do(func() {
+		logger.Debugf("duplicate key error patch applied")
+	})
 	session, err := DialWithTimeout(url, 10*time.Second)
 	if err == nil {
 		session.SetSyncTimeout(1 * time.Minute)