A question about user guide README -> Customer Counter

[Jigao Luo]

My question arises from https://github.com/google/benchmark/blob/main/docs/user_guide.md#custom-counters

```

// This says that we process with the rate of state.range(0) bytes every iteration: state.counters["BytesProcessed"] = Counter(state.range(0), benchmark::Counter::kIsIterationInvariantRate, benchmark::Counter::OneK::kIs1024);

```

I don't get why state.range(0) is related to the number of bytes. It seems a input parameter.