* ADD: add operationName length threshold
* MOD:move operationName threshold to Config, simplified code
* MOD:update agent set-up document, clean code
* MOD:add agent.operation_name_threshold conf prefix
* MOD: Modify compatibility of kafka plugin and expand operationName
* MOD: Delete localSpan layer, modify consumer operationName, delete producer key
* MOD: Modify operationName, reduce String operation
* FIX: Fix callback break, the new version of kafka callback is intercepted twice and needs to be judged
* FIX: Fix different problems with KafkaProducer constructor in old and new versions
* MOD: separate code
* add logstash logback plugin
* add logstash logback plugin, set pom provided, refine logback doc
* refine logstash logback doc
* refine the comment
* remove the app_id that not related to SkyWalking
* remove tag @date for CI error: unknown tag: date
* add description for iLoggingEvent for ci warning: no description for @param
* Support op group name in agent config core.
* Provide core API for operation name grouping, rest template plugin update, and doc update.
* Fix doc
* Fix test case.
* Fix missing changes.
* add shuyun to powered-by.md (#3341)
* add shuyun to powered-by.md
* update powered-by.md
* Remove static method.
* Add cached writer so we need not call whole method.
* Make writer volatile.
* Make writer to WRITER.
* Rewrite the writeFactory for add the cached writer and keep the unit tests stable.
* Give the default writer.
* Remove volatile.
* Stabilize unit test
The FileWriterTest creates files in the source directory(failed to
delete them due to a bug in the `deleteDir` method), making
them being checked by checkstyle plugin and failed to pass.
* Seperate compilation step
* Refactor open edge.
* Make refactor works.
* Revert log back.
* Remove unnecessary codes.
* Open edge for all internal binding interceptors.
* Add document about JDK 9+ module.
* Change bytebuddy to 1.10.1(unreleased)
* Change document
* 1. Adopt the latest bytebudy changes(snapshot only), commit b609711564.
2. Remove disruptor by ArrayBlockingQueue, because Disruptor can't work in both JDK 8(1) and 9-12, if we don't add `--add-modules jdk.unsupported`
3. Doc update.
* Update README.md