=act,tes #15064 Initial MetricsKit draft as well as including it in ActorPerfSpec

The MetricsKit lives next to the AkkaSpec, and is private[akka]

Example output:

```
actor-creation.PropsnewEmptyArgsActorsame_warmup
             count = 50000
         mean rate = 7880.37 calls/second
     1-minute rate = 10000.00 calls/second
     5-minute rate = 10000.00 calls/second
    15-minute rate = 10000.00 calls/second
               min = 1.00 microseconds
               max = 15.00 microseconds
              mean = 3.77 microseconds
            stddev = 1.72 microseconds
            median = 3.00 microseconds
              75% <= 4.75 microseconds
              95% <= 6.00 microseconds
              98% <= 9.00 microseconds
              99% <= 12.00 microseconds
            99.9% <= 14.97 microseconds
```