fix json report

[zhaohuabing]

Nighthawk emits nearby histogram percentiles instead of exact decimal values, for example the benchmark testdata shows values like:

  Percentile  Count   Value
  0.500000    17999   478.255µs
  0.750000    27001   494.671µs
  0.800000    28801   500.831µs
  0.900000    32398   524.479µs
  0.950000    34199   565.567µs
  0.990625    35660   1.174719ms
  0.999023    35962   4.073215ms

The current Json file will output 0 for P99.

        "percentiles": {
          "p50": 0.47825500000000004,
          "p75": 0.494671,
          "p80": 0.500831,
          "p90": 0.524479,
          "p95": 0.565567,
          "p99": 0,
          "p999": 0
        }

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	2656297
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/69ca136e35f4a000080120d7

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.31%. Comparing base (945fe9f) to head (2656297).
⚠️ Report is 85 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8614      +/-   ##
==========================================
- Coverage   74.33%   74.31%   -0.03%     
==========================================
  Files         243      243              
  Lines       38155    38155              
==========================================
- Hits        28364    28354      -10     
- Misses       7802     7809       +7     
- Partials     1989     1992       +3     

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