[Feature]Add metric extension to Sentinel internal statistics

[CarpenterLee]

Describe what this PR does / why we need it

Sentient has various flow control strategies, all these strategies are based on Sentinel internal statistics. The real-time statistics are also useful for system monitoring. Unfortunately, Sentinel does not provide an extension to obtain these data directly.

This PR provides an extension to Sentinel internal statistics.

As discussed in #211, standardizing Sentinel metrics and monitoring is necessary, but we can't expect sentinel to do this. Sentinel should not care about the metric implementation, that is in which format the data is stored or in which way the metric is exposed. That is why MetricExtension is relevant.

In the latter version, it may be necessary to refactor MetricWriter to obey the MetricExtension pattern.

Does this pull request fix one issue?

NONE

Describe how you did it

Add MetricExtension interface and related SPI classes as an extension to Sentinel internal statistics. This extension provides callbacks when a request passes rule checking, blocked by flow control, successfully end or exception occurred. Accompanied by these events, response time and current thread number will be recorded too.

Describe how to verify it

Run test cases.

Special notes for reviews

[codecov-io]

Codecov Report

Merging #730 into master will increase coverage by 0.66%.
The diff coverage is 76.19%.

@@             Coverage Diff              @@
##             master     #730      +/-   ##
============================================
+ Coverage      40.4%   41.06%   +0.66%     
- Complexity     1312     1338      +26     
============================================
  Files           297      301       +4     
  Lines          8591     8630      +39     
  Branches       1156     1159       +3     
============================================
+ Hits           3471     3544      +73     
+ Misses         4695     4661      -34     
  Partials        425      425

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/com/alibaba/csp/sentinel/Tracer.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../sentinel/metric/extension/MetricCallbackInit.java	100% <100%> (ø)	2 <2> (?)
...metric/extension/callback/MetricEntryCallback.java	100% <100%> (ø)	5 <5> (?)
...inel/metric/extension/MetricExtensionProvider.java	76.92% <76.92%> (ø)	4 <4> (?)
.../metric/extension/callback/MetricExitCallback.java	88.88% <88.88%> (ø)	3 <3> (?)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.58% <0%> (+2.94%)	34% <0%> (+1%)	⬆️
... and 4 more

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[sczyh30]

LGTM