Improve benchmark output from vespa-feed-client

[bjorncs]

The extended output format is gated behind an environment variable.

Example:

{
  "feeder.seconds": 730.403,
  "feeder.ok.count": 85858,
  "feeder.ok.rate": 117.549,
  "feeder.error.count": 0,
  "feeder.inflight.count": 277,
  "http.request.count": 3854073,
  "http.request.bytes": 8385743742,
  "http.request.MBps": 11.481,
  "http.exception.count": 0,
  "http.response.count": 3853796,
  "http.response.bytes": 505883404,
  "http.response.MBps": 0.693,
  "http.response.error.count": 3767938,
  "http.response.latency.millis.min": 0.000,
  "http.response.latency.millis.avg": 47.000,
  "http.response.latency.millis.max": 3826.000,
  "operation.latency": {
    "min": 361,
    "avg": 2211,
    "max": 24990
  },
  "http.response": {
    "200": {
      "count": 85858,
      "latency": {
        "min": 197,
        "avg": 1729,
        "max": 3826
      }
    },
    "429": {
      "count": 3767938,
      "latency": {
        "min": 0,
        "avg": 9,
        "max": 461
      }
    }
  }
}

[vekterli]

👍

[bjorncs]

@vekterli PTAL. Last commit is new.

[bjorncs]

Rebasing with master to (hopefully) resolve strange PR build failure