finish `accumulatedAgentTime` work from "onweek2-internal-agent-metrics" branches

For my On Week 2 work I started patches to add counters to internal agent stats for accumulated CPU time used.
For example, these fields from `Agent#_getStats()`:

```
...
agentOverhead: 0.5328552433344271,
accumulatedAgentTime: 941.2509546279907,
cpuUsage: 1766.4290000000037,
agentTimeFromWhat: {
startTransaction: 84.39729332923889,
startSpan: 397.08366441726685,
'Span.end': 23.216548442840576,
'Span._encode': 21.22274398803711,
'Span filter & send': 13.452475309371948,
'Transaction#end': 177.3358075618744,
'apmclient._writeBatch': 224.54242157936096
},
...
}
```

I think with better names for those `agentTimeFromWhat` keys, this could be extremely useful for performance work. For example `agentTimeFromWhat.startSpan` includes the time for `Error.captureStackTrace(obj, Span)` that is relevant for deciding if `captureSpanStackTraces` should be disabled by default (https://github.com/elastic/apm-agent-nodejs/issues/2092).

The code changes are not too much of a burden, I don't think. Starter patches are here:

- https://github.com/elastic/apm-agent-nodejs/compare/trentm/onweek2-internal-agent-metrics/
- https://github.com/elastic/apm-nodejs-http-client/compare/trentm/onweek2-internal-agent-metrics/