[BUG] Setup function running multiple times

**Describe the bug**

According to the [user guide](https://github.com/google/benchmark/blob/8d86026c67e41b1f74e67c1b20cc8f73871bc76e/docs/user_guide.md?plain=1#L258=), global setup and teardown functions should be once, unless using multiple threads:

> The setup/teardown callbacks will be invoked once for each benchmark.
If the benchmark is multi-threaded (will run in k threads), they will be invoked exactly once before
each run with k threads.

However, I'm seeing the setup and teardown functions are being invoked multiple times as the framework determines its iteration count.

**System**
Which OS, compiler, and compiler version are you using:
- OS: Ubuntu 22.04
- Compiler and version: clang++ 14.0.0
- Benchmark version: `8d86026c67e41b1f74e67c1b20cc8f73871bc76e`

**To reproduce**
Steps to reproduce the behavior:
1. Follow the README to build _benchmark_
2. Add a single benchmark using the `BENCHMARK` macro, registering a setup function with `Setup`.
4. Add a print statement to your setup function
5. Run the benchmark single-threaded and see how often the setup function is called

**Expected behavior**
I'd expect the setup function is called once for each benchmark, not each time as the framework determines its iteration count.