plugin factory: remove useless RegisterPlugin log output

[pohly]

RegisterPlugin gets called during the init phase of programs using this package. During init, klog verbosity is usually zero (making the log call redundant because it doesn't print anything).

This could be ignored, but it gets worse when some package increases verbosity during init:

• plugin "systemd" gets registered: no output

• verbosity gets changed

• plugin "containerd" gets registered:

  I1210 14:05:15.799869 126449 factory.go:195] Registered Plugin "containerd"

As a general rule, init code should not log because logging cannot be properly initialized until after the program had a chance to parse command line parameters.

Fixes: #3707

[pohly]

Reproducer from Kubernetes (but with log output only in kubernetes/kubernetes#133844 so far):

kubernetes$ dlv test ./test/e2e -- -test.v -list-tests
Type 'help' for list of commands.
(dlv) b RegisterPlugin
Breakpoint 1 set at 0x4eb7596 for github.com/google/cadvisor/container.RegisterPlugin() ./vendor/github.com/google/cadvisor/container/factory.go:189
(dlv) c
> [Breakpoint 1] github.com/google/cadvisor/container.RegisterPlugin() ./vendor/github.com/google/cadvisor/container/factory.go:189 (hits goroutine(1):1 total:1) (PC: 0x4eb7596)
   184:		// Register is invoked when starting a manager. It can optionally return a container watcher.
   185:		// A returned error is logged, but is not fatal.
   186:		Register(factory info.MachineInfoFactory, fsInfo fs.FsInfo, includedMetrics MetricSet) (watcher.ContainerWatcher, error)
   187:	}
   188:	
=> 189:	func RegisterPlugin(name string, plugin Plugin) error {
   190:		pluginsLock.Lock()
   191:		defer pluginsLock.Unlock()
   192:		if _, found := plugins[name]; found {
   193:			return fmt.Errorf("Plugin %q was registered twice", name)
   194:		}
(dlv) p name
"systemd"
(dlv) bt
0  0x0000000004eb7596 in github.com/google/cadvisor/container.RegisterPlugin
   at ./vendor/github.com/google/cadvisor/container/factory.go:189
1  0x0000000004f8cb7d in github.com/google/cadvisor/container/systemd/install.init.0
   at ./vendor/github.com/google/cadvisor/container/systemd/install/install.go:26
2  0x0000000001bd38f4 in runtime.doInit1
   at /nvme/gopath/go-1.25.2/src/runtime/proc.go:7670
3  0x0000000001bfe1aa in runtime.doInit
   at /nvme/gopath/go-1.25.2/src/runtime/proc.go:7637
4  0x0000000001bc4e9e in runtime.main
   at /nvme/gopath/go-1.25.2/src/runtime/proc.go:256
5  0x0000000001c05a41 in runtime.goexit
   at /nvme/gopath/go-1.25.2/src/runtime/asm_amd64.s:1693
(dlv) c

> [Breakpoint 1] github.com/google/cadvisor/container.RegisterPlugin() ./vendor/github.com/google/cadvisor/container/factory.go:189 (hits goroutine(1):2 total:2) (PC: 0x4eb7596)
   184:		// Register is invoked when starting a manager. It can optionally return a container watcher.
   185:		// A returned error is logged, but is not fatal.
   186:		Register(factory info.MachineInfoFactory, fsInfo fs.FsInfo, includedMetrics MetricSet) (watcher.ContainerWatcher, error)
   187:	}
   188:	
=> 189:	func RegisterPlugin(name string, plugin Plugin) error {
   190:		pluginsLock.Lock()
   191:		defer pluginsLock.Unlock()
   192:		if _, found := plugins[name]; found {
   193:			return fmt.Errorf("Plugin %q was registered twice", name)
   194:		}
(dlv) p name
"crio"
(dlv) c

> [Breakpoint 1] github.com/google/cadvisor/container.RegisterPlugin() ./vendor/github.com/google/cadvisor/container/factory.go:189 (hits goroutine(1):3 total:3) (PC: 0x4eb7596)
   184:		// Register is invoked when starting a manager. It can optionally return a container watcher.
   185:		// A returned error is logged, but is not fatal.
   186:		Register(factory info.MachineInfoFactory, fsInfo fs.FsInfo, includedMetrics MetricSet) (watcher.ContainerWatcher, error)
   187:	}
   188:	
=> 189:	func RegisterPlugin(name string, plugin Plugin) error {
   190:		pluginsLock.Lock()
   191:		defer pluginsLock.Unlock()
   192:		if _, found := plugins[name]; found {
   193:			return fmt.Errorf("Plugin %q was registered twice", name)
   194:		}
(dlv) p name
"containerd"
(dlv) so
I1210 15:45:58.259803  396947 factory.go:195] Registered Plugin "containerd"
> github.com/google/cadvisor/container/containerd/install.init.0() ./vendor/github.com/google/cadvisor/container/containerd/install/install.go:26 (PC: 0x4f8675d)

Only "containerd" is logged.

The output is also distracting:

kubernetes$ go test -v ./test/e2e -args -list-tests
I1210 15:48:13.482534  399943 factory.go:195] Registered Plugin "containerd"
The following spec names can be used with 'ginkgo run --focus/skip':
    apimachinery/watchlist.go:258: [sig-api-machinery] API Streaming (aka. WatchList) [FeatureGate:WatchList] [Beta] reflector doesn't support receiving resources as Tables
...

cc @dims