Record max concurrency in MetricItem and metric logs

[liqiangz]

Describe what this PR does / why we need it

Record max concurrency in MetricItem and metric logs

Does this pull request fix one issue?

Fixes #349

Describe how you did it

Describe how to verify it

Special notes for reviews

[codecov-io]

Codecov Report

Merging #371 (7c3ab9c) into master (c059530) will increase coverage by 0.31%.
The diff coverage is 52.54%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   50.20%   50.52%   +0.31%     
==========================================
  Files          70       70              
  Lines        4089     4190     +101     
==========================================
+ Hits         2053     2117      +64     
- Misses       1759     1783      +24     
- Partials      277      290      +13     

Impacted Files	Coverage Δ
core/stat/base/bucket_leap_array.go	57.57% <41.17%> (-1.89%)	⬇️
core/stat/base/sliding_window_metric.go	61.53% <55.55%> (-0.87%)	⬇️
core/stat/base/metric_bucket.go	77.77% <100.00%> (+6.34%)	⬆️
core/flow/rule_manager.go	61.09% <0.00%> (+1.03%)	⬆️

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[luckyxiaoqiang]

LGTM

[binbin0325]

LGTM