* Add `@Spi` annotation as the general annotation for SPI definition.
* Add isDefault in @Spi, add loadDefaultInstance and improve loadFirstInstanceOrDefault method, improve test cases
* Add SpiLoaderException class for thrown when something goes wrong while loading Provider
* Rearrange packages of base SPI mechanism

NOTE: this PR contains breaking changes regarding API.

Signed-off-by: Jason Joo <hblzxsj@163.com>

* Optimize logging statements using placeholder to avoid unnecessary concatenation (issue #1735)

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Unify the extended interface as a few event handlers: onPass, onBlocked, onComplete and onError
- Polish related code

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Add EntryType args to all hook methods

- Rename: whenComplete -> whenTerminate
- Execute the exit handler directly after the onExit hook of all slots

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- Only carry context in tryPass/onComplete method (this might be generic in upcoming versions)

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Refactor the workflow to fix the bug that circuit breaker may remain half-open state forever when the request is blocked by upcoming rules: revert the state change in exit handler (as a temporary workaround)
* Add exit handler in Entry as a per-invocation hook.

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Add `CircuitBreaker` abstraction (with half-open state) and add circuit breaker state change event observer support.
* Improve circuit breaking strategy (avg RT → slow request ratio) and make statistics of each rule dependent (to support arbitrary statistic interval).
* Add simple "trial" mechanism (aka. half-open).
* Refactor mechanism of metric recording and state change handling for circuit breakers: record RT and error when requests have completed (i.e. `onExit`, based on #1420).

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Also polish related complete callbacks

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

- which may lead to IllegalMonitorStateException in unlock() when unchecked error occurs during lock()

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Improve log info in SpiLoader, improve comment and test case
* Use error level in catch block, init ArrayList with capacity and improve add item to list

* Note that this is only a temporary solution.

* Make slots loaded by SPI, mark all slots with @SpiOrder from -10000 to -1000, improve comment
* Reserve gateway and param slot chain builder (just extends DefaultSlotChainBuilder) and mark them as @Deprecated

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Polish placeholders in logging content to "{}"

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* Move the legacy JUL methods from LogBase to BaseJulLogger.
* Add a JavaLoggingAdapter as the general JUL adapter for the Logger SPI, which makes it convenient to use (as the default logger).
* Add LoggerSpiProvider to resolve Logger SPI.
* Polish the logback demo.

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* There are two types of logger for command center and common modules specified by  annotation of `LogTarget`
* Add implementing examples in `sentinel-demo/sentinel-demo-log-logback`
* All implementations should support placeholder '{}'