Update unmanaged_mysql_sysbench_benchmark to output single-threaded latency as a new metric

In unmanaged_mysql_sysbench_benchmark, we loop through a range of thread counts to find the transactions 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 max transactions achieve across all the thread counts as a new metric. i.e.

max_transaction_tps = 0
max_transaction_qps = 0
for thread_count in FLAGS.sysbench_run_threads:
...
results += sysbench.ParseSysbenchTransactions(stdout, metadata)
assert results[0].metric = 'tps'
assert results[1].metric = 'qps'
current_tps = results[0].value
current_qps = results[1].value
max_transaction_tps = max(current_tps, max_transaction_tps)
max_transaction_qps = max(current_qps, max_transaction_qps)