clientv3/integration: add put blackhole test

[xiang90]

/cc @gyuho

[xiang90]

/cc @jpbetz

[gyuho]

This cancel is overwritten by line 68?

package main

import (
	"context"
	"fmt"
	"time"
)

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
	_ = ctx
	defer func() {
		fmt.Println("cancel 1:", cancel)
		cancel()
	}()

	ctx, cancel = context.WithTimeout(context.Background(), time.Second)
	_ = ctx
	defer func() {
		fmt.Println("cancel 2:", cancel)
		cancel()
	}()
}

/*
cancel 2: 0x109a950
cancel 1: 0x109a950
*/

Should we move cancel to line 65?

[xiang90]

will fix.

[gyuho]

lgtm. Confirmed locally that test passes. Semaphore might take awhile due to backlog.

[gyuho]

SkipCreatingClient: true,?

[jpbetz]

Code looks good.

Are there any references to issues or ongoing gPRC work that we can reference from this PR? It might be helpful in the future to have some breadcrumbs tying the gPRC LB work/issues together.

[xiang90]

failfast issue: grpc/grpc-go#1463
new balancer work: grpc/grpc-go#1388

testing plan for current balancer: #8711

/cc @jpbetz let me know if you want more reference.

[jpbetz]

@xiang90 That's plenty. Thanks!