This logr implementation can be used in tests to ensure that output
is associated with the currently running test. Compared to using the
global klog instance in a standard Go test that has some advantages:
- Log messages are associated with the currently running test.
  Tests can run in parallel without interleaving their output.
- Log output is only printed when a test fails, unless "go test -v"
  is used.
- Because of that, the logger can print more log messages by default,
  with 5 being used because that is recommended for debugging in
  https://github.com/kubernetes/community/blob/fbc5ca53d6bfd8388b335f7d0198b67a14b99d91/contributors/devel/sig-instrumentation/logging.md?plain=1#L34-L36
  That threshold can be changed via -testing.v after registering
  that flag by importing k8s.io/klog/v2/ktesting/init.

The motivation for hosting this logger in klog is that it shares the formatting
code with klogr. Conceptually this is identical to go-logr/logr/testing, just
the output is different (klog text format vs. JSON).

  $ go test -v ./testinglogger/example/
  === RUN   TestKlog
      example_test.go:69: INFO hello world
      example_test.go:70: ERROR failed err="failed: some error"
      example_test.go:71: INFO verbosity 1
      example_test.go:72: INFO main/helper: with prefix
      example_test.go:73: INFO key/value pairs int=1 float=2 pair="(1, 2)" kobj="kube-system/sally"
  --- PASS: TestKlog (0.00s)

The corresponding output from go-logr/logr/testing would be:

  === RUN   TestLogr
      example_test.go:69: "ts"="2021-09-07 16:44:54.307551" "level"=0 "msg"="hello world"
      example_test.go:70: "ts"="2021-09-07 16:44:54.307664" "msg"="failed" "error"="failed: some error"
      example_test.go:71: "ts"="2021-09-07 16:44:54.307686" "level"=1 "msg"="verbosity 1"
      example_test.go:72: main/helper: "ts"="2021-09-07 16:44:54.307703" "level"=0 "msg"="with prefix"
      example_test.go:73: "ts"="2021-09-07 16:44:54.307733" "level"=0 "msg"="key/value pairs" "int"=1 "float"=2 "pair"={} "kobj"={"name":"sally","namespace":"kube-system"}
  --- PASS: TestLogr (0.00s)

The goal is to have only "legacy" code in klog.go.

As discussed in kubernetes/enhancements#3078, enabling
code that only imports klog for all logging makes life simpler for developers.

This was also the reason why KObj was added to klog instead of a separate
helper package.

This moves the global logger into a separate file together with the functions
that implement contextual logging.

The advantage of SetLoggerWithOptions over alternatives like a
SetContextualLogger without options is that it can be used as a drop-in
replacement for SetLogger. This allows us to deprecate SetLogger. Whether we
actually do that is kept open for now.