add memusage stat to os_provider and use it in benchmarks

[lplewa]

Description

Checklist

• Code compiles without errors locally
• All tests pass locally
• CI workflows execute properly

• CI workflows, not executed per PR (e.g. Nightly), execute properly
• New tests added, especially if they will fail without my changes
• Added/extended example(s) to cover this functionality
• Extended the README/documentation
• All newly added source files have a license
• All newly added source files are referenced in CMake files
• Logger (with debug/info/... messages) is used
• All API changes are reflected in docs and def/map files, and are tested

[vinser52]

Why do you need to load os_provider->stats.allocated_memory again, instead of using the result of the atomic_fetch_add_explicit on the line above?

[lplewa]

done

[KFilipek]

Suggested change

	memused.resize(state.threads());
	for (int i = 0; i < state.threads(); i++) {
	memused[i] = 0;
	}
	memused.assign(state.threads(), 0);

or

Suggested change

	memused.resize(state.threads());
	for (int i = 0; i < state.threads(); i++) {
	memused[i] = 0;
	}
	memused.resize(state.threads());
	std::fill(memused.begin(), memused.end(), 0);

[KFilipek]

Suggested change

	size_t current_memory_allocated = 0;
	for (int i = 0; i < state.threads(); i++) {
	current_memory_allocated += memused[i];
	}
	size_t current_memory_allocated = 0;
	for (const auto &used : memused) {
	current_memory_allocated += used;
	}