[augerctl] Support watch for get

[wzshiming]

part of #3

Test

Preparation cluster

Create a cluster and expose etcd port, to facilitate the creation of a cluster using kwokctl, this can be any other k8s cluster

# brew install kwok
kwokctl create cluster --etcd-port 2379

Watch leases changes

augerctl get leases -n kube-system -w

[wzshiming]

/cc @jmhbnz
/cc @siyuanfoundation

[jmhbnz]

Hey @wzshiming - I'll review this one next, can you please rebase to fix conflicts, thanks! 🙏🏻

[wzshiming]

Done, thank you for reviewing it 😄

[jmhbnz]

Hey @wzshiming - General question, have you put any thought into potential approaches for writing tests for augerctl? To add new features it gets easier over time if we have tests to verify existing features don't break.

[wzshiming]

Sure, my original plan was to add the e2e test after the subcommand get.

[hwdef]

@wzshiming
Hi, will you continue doing this PR?

[k8s-ci-robot]

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[Copilot]

Pull request overview

Adds watch support to augerctl get, enabling streaming updates from etcd (similar to kubectl get -w) as part of the broader effort to provide kubectl-like direct-etcd operations.

Changes:

• Extend the etcd client abstraction with Watch, plus a WithRevision option and PrevValue support in KeyValue.
• Add --watch/-w and --watch-only flags to augerctl get, wiring them to the new watch implementation.
• Update YAML/JSON printers to fall back to printing PrevValue when the current value is absent (e.g., delete events), and document watch usage.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
pkg/client/client.go	Adds Watch to the interface, introduces WithRevision, and extends KeyValue with PrevValue.
pkg/client/client_watch.go	New etcd watch implementation that streams events into the existing response callback.
augerctl/command/get_command.go	Adds CLI flags and control flow to list-then-watch or watch-only.
augerctl/command/printer_yaml.go	Prints PrevValue when Value is missing during watch output.
augerctl/command/printer_json.go	Prints PrevValue when Value is missing during watch output.
augerctl/README.md	Documents augerctl get ... -w usage and kubectl equivalence guidance.

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[Copilot]

--watch-only currently requires --watch (and otherwise errors), which differs from kubectl semantics and makes --watch-only awkward to use. Consider making --watch-only imply watch mode (i.e., treat --watch-only as equivalent to --watch --watch-only) and enforce mutual exclusion in flag validation instead of requiring both flags.