Flow control by frequent (hot spot) parameters

[sczyh30]

Describe what this PR does / why we need it

In some circumstances, users may want to do flow control by frequent (hot spot) parameters. For example:

• userId as parameter, limit top-frequent users
• productId as parameter, limit invocations on relevant services for top-N products

Like normal flow control, flow control by frequent parameters support two mode: QPS (default) and thread count.

Does this pull request fix one issue?

Closes #32

Describe how you did it

• Add a new module sentinel-parameter-flow-control under sentinel-extension
• Add a CacheMap interface to provide abstraction for cache. We use ConcurrentLinkedHashMap as the default implementation (LRU strategy).
• Add a ParameterMetric class as frequent parameter metrics for a specific resource. The metric map is located in ParamFlowSlot rather than ClusterNode
• Implement ParameterLeapArray as statistic data structure for frequent parameters in a period of time window
• Add ParamFlowSlot as the checker slot; Add ParamFlowChecker to do rule checking; Add ParamFlowRuleManager to do rule managing
• The statistic metrics for frequent parameters is enabled only if the related resource has configured parameter flow rule.
• Add callback registry for statistic slot for extensions
• Leverage extensible SlotChainBuilder to provide a HotParamSlotChainBuilder

Describe how to verify it

See test cases and demo.

[codecov-io]

Codecov Report

Merging #156 into master will increase coverage by 1.47%.
The diff coverage is 54.65%.

@@             Coverage Diff              @@
##             master     #156      +/-   ##
============================================
+ Coverage     50.01%   51.49%   +1.47%     
- Complexity      690      878     +188     
============================================
  Files           122      141      +19     
  Lines          4213     5166     +953     
  Branches        577      770     +193     
============================================
+ Hits           2107     2660     +553     
- Misses         1867     2207     +340     
- Partials        239      299      +60

Impacted Files	Coverage Δ	Complexity Δ
...ots/statistic/ParamFlowStatisticEntryCallback.java	0% <0%> (ø)	0 <0> (?)
...tinel/init/ParamFlowStatisticSlotCallbackInit.java	0% <0%> (ø)	0 <0> (?)
...nd/handler/ModifyParamFlowRulesCommandHandler.java	0% <0%> (ø)	0 <0> (?)
.../command/handler/FetchTopParamsCommandHandler.java	0% <0%> (ø)	0 <0> (?)
...mmand/handler/GetParamFlowRulesCommandHandler.java	0% <0%> (ø)	0 <0> (?)
...a/csp/sentinel/slots/HotParamSlotChainBuilder.java	0% <0%> (ø)	0 <0> (?)
.../sentinel/slots/statistic/data/ParamMapBucket.java	100% <100%> (ø)	8 <8> (?)
...inel/slots/block/flow/param/RollingParamEvent.java	100% <100%> (ø)	1 <1> (?)
...ba/csp/sentinel/slots/statistic/StatisticSlot.java	61.11% <11.11%> (-11.62%)	10 <0> (ø)
...slots/statistic/StatisticSlotCallbackRegistry.java	22.22% <22.22%> (ø)	3 <3> (?)
... and 46 more

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[CarpenterLee]

LGTM