Rework benchmark tool

[ijsong]

There is already a benchmark-like tool in cmd/stress, but we need to rework it for the following reasons:

• Only AppendTo RPC is supported, but various RPC results such as Append and Subscribe are often required.
• It only reports the full metrics since the start of the benchmark; however, it cannot report the latest metrics.
• Expanding metrics is cumbersome. For example, adding a new metric requires a lot of code to define and report the metric.

In addition to the above, the old tool has more to improve.