Also make some design changes exposed in testing and review.

Do not remove the ambiguous old metric
`apiserver_flowcontrol_request_concurrency_limit` because reviewers
though it is too early.  This creates a problem, that metric can not
keep both of its old meanings.  I chose the configured concurrency
limit.

Testing has revealed a design flaw, which concerns the initialization
of the seat demand state tracking.  The current design in the KEP is
as follows.

> Adjustment is also done on configuration change … For a newly
> introduced priority level, we set HighSeatDemand, AvgSeatDemand, and
> SmoothSeatDemand to NominalCL-LendableSD/2 and StDevSeatDemand to
> zero.

But this does not work out well at server startup.  As part of its
construction, the APF controller does a configuration change with zero
objects read, to initialize its request-handling state.  As always,
the two mandatory priority levels are implicitly added whenever they
are not read.  So this initial reconfig has one non-exempt priority
level, the mandatory one called catch-all --- and it gets its
SmoothSeatDemand initialized to the whole server concurrency limit.
From there it decays slowly, as per the regular design.  So for a
fairly long time, it appears to have a high demand and competes
strongly with the other priority levels.  Its Target is higher than
all the others, once they start to show up.  It properly gets a low
NominalCL once other levels show up, which actually makes it compete
harder for borrowing: it has an exceptionally high Target and a rather
low NominalCL.

I have considered the following fix.  The idea is that the designed
initialization is not appropriate before all the default objects are
read.  So the fix is to have a mode bit in the controller.  In the
initial state, those seat demand tracking variables are set to zero.
Once the config-producing controller detects that all the default
objects are pre-existing, it flips the mode bit.  In the later mode,
the seat demand tracking variables are initialized as originally
designed.

However, that still gives preferential treatment to the default
PriorityLevelConfiguration objects, over any that may be added later.

So I have made a universal and simpler fix: always initialize those
seat demand tracking variables to zero.  Even if a lot of load shows
up quickly, remember that adjustments are frequent (every 10 sec) and
the very next one will fully respond to that load.

Also: revise logging logic, to log at numerically lower V level when
there is a change.

Also: bug fix in float64close.

Also, separate imports in some file

Co-authored-by: Han Kang <hankang@google.com>