proxy: improve log for retrying an unavailable endpoint
proxy: improve log for retrying an unavailable endpoint

Fixes #3541

Signed-off-by: Guohua ouyang <93b34e674692128999fc615c198b75f23de03cef@gmail.com>

rnaveiras/etcd,cchamplin/etcd,kkaneda/etcd,pauldallen/etcd,ajityagaty/etcd,xychu/etcd,combk8s/etcd,linuxcer/etcd,xiang90/etcd,fasaxc/etcd,bluebreezecf/etcd,raoofm/etcd,jonboulle/etcd,sinsharat/etcd,wowsoso/etcd,StorReduce/etcd,yichengq/etcd,luxas/etcd,jonboulle/etcd,cockroachdb/etcd,akauppi/etcd,shawnps/etcd,li-ang/etcd,dannysauer/etcd,mkumatag/etcd,ajityagaty/etcd,sofuture/etcd,visheshnp/etcd,ravigadde/etcd,YuleiXiao/etcd,zaphod-concur/etcd,gouyang/etcd,chipironcin/etcd,westhood/etcd,rchicoli/etcd,OyTao/etcd_learning,ravigadde/etcd,linuxcer/etcd,siddontang/etcd,bdarnell/etcd,cchamplin/etcd,rchicoli/etcd,oscarrr110/etcd,Timer/etcd,wowsoso/etcd,mkumatag/etcd,vimalk78/etcd,yudai/etcd,fanminshi/etcd,fasaxc/etcd,dannysauer/etcd,gouyang/etcd,achanda/etcd,jeremyeder/etcd,HideoYamauchi/etcd,tguangch/etcd,chipironcin/etcd,wantonsolutions/dinv-etcd,glevand/coreos--etcd,hongchaodeng/etcd,insequent/etcd,bdarnell/etcd,handsomesun/etcd,sebrandon1/etcd,whuwxl/etcd,jlamillan/etcd,swingbach/etcd,improbable-io/etcd,yudai/etcd,siddontang/etcd,YuleiXiao/etcd,fanminshi/etcd,qqsun8819/etcd,sayden/etcd,asim/etcd,joshix/etcd,tamird/etcd,gyuho/etcd,CrazyJvm/etcd,CrazyJvm/etcd,shawnps/etcd,kkaneda/etcd,dnaeon/etcd,endocode/etcd,purpleidea/etcd,purpleidea/etcd,lorneli/etcd,cockroachdb/etcd,gyuho/etcd,ktateish/etcd,xiang90/etcd,vimalk78/etcd,lumjjb/etcd,akauppi/etcd,glycerine/etcd,OyTao/etcd_learning,a-robinson/etcd,dnaeon/etcd,HideoYamauchi/etcd,fasaxc/etcd,plasticbox/etcd,StorReduce/etcd,whuwxl/etcd,visheshnp/etcd,mqliang/etcd,sofuture/etcd,swingbach/etcd,ajityagaty/etcd,joshix/etcd,nekto0n/etcd,achanda/etcd,armstrongli/etcd,lumjjb/etcd,bluebreezecf/etcd,sebrandon1/etcd,rnaveiras/etcd,heyitsanthony/etcd,plasticbox/etcd,coreos/etcd,lorneli/etcd,mickep76/etcd,wantonsolutions/dinv-etcd,tguangch/etcd,handsomesun/etcd,insequent/etcd,sinsharat/etcd,etcd-io/etcd,coreos/etcd,combk8s/etcd,improbable-io/etcd,yichengq/etcd,rnaveiras/etcd,endocode/etcd,glevand/coreos--etcd,ktateish/etcd,zaphod-concur/etcd,Timer/etcd,mqliang/etcd,hongchaodeng/etcd,glycerine/etcd,etcd-io/etcd,mkumatag/etcd,luxas/etcd,raoofm/etcd,tamird/etcd,armstrongli/etcd,li-ang/etcd,jlamillan/etcd,wantonsolutions/dinv-etcd,qqsun8819/etcd,heyitsanthony/etcd,etcd-io/etcd,mickep76/etcd,westhood/etcd,xychu/etcd,a-robinson/etcd,nekto0n/etcd,oscarrr110/etcd,sayden/etcd,jeremyeder/etcd,asim/etcd,pauldallen/etcd,mqliang/etcd