client/v3: add MemberWatcher helper for polling-based member changes

[MkDev11]

What this PR does

This adds a client-side utility for watching membership changes by polling MemberList() and comparing with the previous state.

Fixes: #5277

Why we need it

Users want to watch for membership changes in etcd v3. Following maintainer feedback, this implements the functionality on the client side rather than adding server complexity.

This approach uses the existing Sync() and MemberList() APIs as suggested by @ahrtr.

Changes

Added single file: client/v3/member_watch.go

• MemberWatcher struct with configurable poll interval
• MemberEvent and MemberEventType types
• Watch() method that returns a channel of membership events

Usage

watcher := clientv3.NewMemberWatcher(client, clientv3.MemberWatchConfig{
    PollInterval: 5 * time.Second,
})

ctx, cancel := context.WithCancel(context.Background())
defer cancel()

for event := range watcher.Watch(ctx, true) {
    switch event.Type {
    case clientv3.MemberAdded:
        fmt.Printf("Member added: %s\n", event.Member.Name)
    case clientv3.MemberRemoved:
        fmt.Printf("Member removed: %s\n", event.Member.Name)
    case clientv3.MemberUpdated:
        fmt.Printf("Member updated: %s\n", event.Member.Name)
    }
}

Event types

Event	Triggered when
MemberAdded	New member detected in cluster
MemberRemoved	Member no longer in cluster
MemberUpdated	Member's peer URLs changed

Test Results

$ go run test_member_watcher.go

Connected to etcd
Starting MemberWatcher (polling every 3s)...
Initial members will be sent as MemberAdded events
Waiting 15 seconds for events...

[Event #1] Type: ADDED
  Member ID:    8e9e05c52164694d
  Member Name:  default
  PeerURLs:     [http://localhost:2380]
  ClientURLs:   [http://localhost:2379]
  IsLearner:    false

Watch ended. Total events received: 1

Testing checklist

• Builds successfully (go build ./client/v3/...)
• Lint passes
• Manual testing verified

[k8s-ci-robot]

Hi @MkDev11. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[MkDev11]

/assign @MkDev11

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: MkDev11
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[MkDev11]

@ahrtr could you please have a look at the PR and let me know your feedback? thanks.

[ahrtr]

This approach uses the existing Sync() and MemberList() APIs as suggested by @ahrtr.

Sorry, I should be clearer. The comment #5277 (comment) is just a workaround solution for users reference. I didn't mean to add it into etcd's client SDK.