Update unmanaged_mysql_sysbench_benchmark to output max TPS and max QPS achieved across all threads as a new metric

[jellyfishcake]

In unmanaged_mysql_sysbench_benchmark, we loop through a range of thread counts to find the average latency for each thread count, and store them as samples: https://github.com/GoogleCloudPlatform/PerfKitBenchmarker/blob/master/perfkitbenchmarker/linux_benchmarks/unmanaged_mysql_sysbench_benchmark.py#L266

This task is about recording the latency achieved at 1 thread as a new metric. if thread_count==1, store the average_latency as a new metric called single-threaded latency.