Memory reporting capabilities are unclear and undocumented.

I am currently using google/benchmark to benchmark https://github.com/protocolbuffers/upb. I would very much like for those benchmarks to also include memory usage.

Does this project support memory benchmarking? The [user guide](https://github.com/google/benchmark/blob/main/docs/user_guide.md) does not mention memory usage benchmarking at all. However I notice that the header has [a `RegisterMemoryManager()` function](https://github.com/google/benchmark/blob/dc1a97174d5b84b1ce6720f1c98f85e6031b4723/include/benchmark/benchmark.h#L302-L304), and there is [a unit test](https://github.com/google/benchmark/blob/main/test/memory_manager_test.cc) verifying this functionality.

From running the test, it appears that the memory usage data is surfaced to JSON, but not CSV or console reports. Is this an oversight or intentional? Ideally this data would be shown on CSV and console reports also.