They are basically proto-version of custom user counters.
It does not seem that they do anything that custom user counters
don't do. And having two similar entities is not good for generalization.

Migration plan:
* ```
  SetItemsProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate)},
    ...
  });
  ```
* ```
  SetBytesProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate,
                                  benchmark::Counter::OneK::kIs1024)},
    ...
  });
  ```
* ```
  <Name>_processed()
    =>
  state.counters["<Name>"]
  ```

One thing the custom user counters miss is better support
for units of measurement.

Refs. https://github.com/google/benchmark/issues/627