[BUG] "MetricObjective" needs to be plumbed through algorithms in cuvs-bench

[cjnolet]

It looks like when cuvs-bench was migrated from RAFT, the metric objective (latency vs throughput) was broken. See this line of code, which was added to cuvs-bench but doesn't exist in the raft-ann-bench, for example. We need to fix this.

cc @achirkin @divyegala

[achirkin]

I believe the bench_mode_ variable in HNSW is not needed once #724 is merged. The only other algorithm that has such a variable is diskann, but it is also not used there at all.

In any case, this information can be obtained in any algorithm by comparing cuvs::bench::benchmark_n_threads global variable to 1.