stderrthreshold is useless if logtostderr is set

[shsjshentao]

stderrthreshold is useless if logtostderr is set.
This pr can ignore logs which log level is under stderrthreshold.
Like:
flag.Set("logtostderr","true")
flag.Set("stderrthreshold","ERROR")

Does this PR introduce a user-facing change?:

The flag "stderrthreshold" will take effect if flag "logtostderr" is set to "true".
In previous version, it will print all level logs whatever "stderrthreshold" is set.

[shsjshentao]

I signed it

[dims]

/approve

[justinsb]

I think we should make alsologtostderr behave the same way?

[justinsb]

I agree with this change, but this is a behavioural change. I don't know if it will catch anyone out, as the new behaviour feels more correct. We should ensure this makes the release notes though!

[DirectXMan12]

I agree as well. I think the previous behavior was more confusing, and I've talked to plenty of people that assume it works this way before trying it (I assumed this as well at some point).

[dims]

@shsjshentao can you please update the Does this PR introduce a user-facing change?: section above?

/approve

[dims]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, shsjshentao

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[dims]

thanks @shsjshentao