* add cassandra java driver 3.x plugin

* Fill my FAQ reply in official document.

* Add image.

* Add TTL E2E test

* Add to Jenkins stage and minor bugfix

* Upgrade e2e container version

* Polish and minor fix

* Polish

* support es https ssl config.

* update

* update docs.

* refactoring var name.

* fix

* update docs.

* add config in es section.

* fix logical.

* Add envoy metrics sending example
Signed-off-by: NDhi Aurrahman <dio@tetrate.io>

* Newline
Signed-off-by: NDhi Aurrahman <dio@tetrate.io>

* Fix
Signed-off-by: NDhi Aurrahman <dio@tetrate.io>

* Tear down note
Signed-off-by: NDhi Aurrahman <dio@tetrate.io>

* Typo
Signed-off-by: NDhi Aurrahman <dio@tetrate.io>

* Update document.

* Enable ALS document

* Support Gateway without agent

* Update powered-by.md

* 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

* Fix the zookeeper plugin has many nodes in the cluster case

* support version

* Support play 2.x 

* 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

* 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.

* Log4j2 support AsyncRoot print tid
Support AsyncRoot print tid of all versions of log4j2 2.1+

* Update powered-by.md

* Update powered-by.md

Change order

* support ACL

* support digest only

* Add pattern logger to replace the easy logger.
* Doc update
* Support console log 

change `agent.span_limit_per_segment` default value to match the default value of `SPAN_LIMIT_PER_SEGMENT` in https://github.com/apache/skywalking/blob/master/apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/conf/Config.java

* improve auto connect

* add config

* use int instread of atomic

* update format

* update doc

* update var name and doc

* fix

* support elasticsearch address configuration with http scheme.

* update logical.

* Remove the meta file

* Add dependency licenses check 

* Update backend-storage.md

* Update backend-storage.md

* Update ES-Server-FAQ.md

* add http

* update plugin

* Add StaticMethodsAroundInterceptor to be injected.

* move to options plugin

* move folder

* update pom

* support https

* add witness class

* Make JDK9-11 available for bootstrap instrumentation

* Remove temp folder of old bootstrap instrumentation requirement.

* remove witness ,work for jdk6,7,8,9,10,11

* Make JDK 12 works.

* move jdk-http-plugin to bootstrap-plugins folder

* add httpsclient

* update README.md

* Support upgrade backend w/o rebooting agents

* Make JDK9-11 available for bootstrap instrumentation

* Remove temp folder of old bootstrap instrumentation requirement.

* correct 3rd default alarm rule description

* unit change: 1000ms -> 1s

* Remove javassist dependency

*   Finish the PR.

* Change doc title level.

* Remove the duplicated codes.

* POC: Intercept successfully.

* Refactor codes

* Build the new core

* Setup the new core.

* Push the javassist version to 3.23.2-GA, as it is the last version supports JDK 1.6 compile, according to https://github.com/jboss-javassist/javassist/blob/rel_3_23_2_ga/pom.xml#L149-L160

* Finish the all codes.

* Document updated for bootstrap instrumentation.

* Add a notice to document.

* Remove unexpected file.

* Set javassist version right. Add judge in core interceptors

* Fixe no log bridge implementation

* First commit

* Fix e2e test and some nits

* Documenting so11y and setting default values of it in dev env