* 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 changelog for 6.4.0 release.

* Update CHANGES.md

ES storage column type changed for pxx details. It wouldn't really 100% compatible.

UI sync

* 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

* sync skywalking-ui

* Avoid unnecessary resetting when service mapping id is already 0

* Avoid as early as possible and minor optimization

* Remove unused import

* Polish

* 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+

*  fix "DATA_BINARY" limit VARCHAR(20000)

*  slow sql long string trim

*  fix slow sql has long string

* Add more configuration and explainations.

* 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.