Improve JSON output produced by olly gc-stats

[punchagan]

1. Fixes incorrect rounded off max_latency and mean_latency values.
2. Includes the percentile values in the latency distribution.

[kayceesrk]

Thanks @punchagan. Would you be able to include a "before-and-after" output here? It would help me review the PR quickly.

[punchagan]

Here are the before and after outputs, both passed through jq for readability

Before

{
  "mean_latency": 0,
  "max_latency": 1,
  "distr_latency": [
    0.291071,
    0.647679,
    0.672255,
    0.693247,
    0.700927,
    0.743935,
    0.794623,
    0.854527,
    1.132543,
    1.175551,
    1.175551,
    1.300479,
    1.300479,
    1.697791,
    1.697791,
    1.697791,
    1.697791,
    1.697791
  ]
}

After

{
  "mean_latency": 0.582986,
  "max_latency": 1.669119,
  "distr_latency": {
    "25.0000": 0.286207,
    "50.0000": 0.657919,
    "60.0000": 0.675839,
    "70.0000": 0.683007,
    "75.0000": 0.698879,
    "80.0000": 0.699903,
    "85.0000": 0.762367,
    "90.0000": 0.893951,
    "95.0000": 1.150975,
    "96.0000": 1.172479,
    "97.0000": 1.172479,
    "98.0000": 1.639423,
    "99.0000": 1.639423,
    "99.9000": 1.669119,
    "99.9900": 1.669119,
    "99.9990": 1.669119,
    "99.9999": 1.669119,
    "100.0000": 1.669119
  }
}

[kayceesrk]

Thanks!