benchmark

State: Initialize counters with kAvgIteration in constructor (#1652)

* State: Initialize counters with kAvgIteration in constructor

Previously, `counters` was updated in `PauseTiming()` with
`counters[name] += Counter(measurement, kAvgIteration)`.

The first `counters[name]` call inserts a counter with no flags.

There is no `operator+=` for `Counter`, so the insertion is done
by converting the `Counter` to a `double`, then constructing a
`Counter` to insert from the `double`, which drops the flags.

Pre-insert the `Counter` with the correct flags, then only
update `Counter::value`.

Introduced in 1c64a36 ([perf-counters] Fix pause/resume (#1643)).

* perf_counters_test.cc: Don't divide by iterations

Perf counters are now divided by iterations, so dividing again
in the test is wrong.

* State: Fix shadowed param error

* benchmark.cc: Fix clang-tidy error

---------

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>