* Feature of database session

* Make it configurable.

* Change the metrics process flow.
before: metrics entrance -> aggregate worker -> remote worker -> trans worker -> minute, hour, day, month persistence worker -> storage
after: metrics entrance -> aggregate worker -> remote worker -> minute persistence worker ->  trans worker -> hour, day, month persistence worker -> storage

* IntKeyLongValueHashMap instead of IntKeyLongValueArray.

* Make the OAP server can't startup.

* Finish

* Rename the method and fixed some test case issues.

* Rename field.

* no message

If transform throw exception, the stream observer won't call the onCompleted method, then the stream observer will wait it finish until the timeout. (#3163)

* Feature of database session

* Make it configurable.

* Change the metrics process flow.
before: metrics entrance -> aggregate worker -> remote worker -> trans worker -> minute, hour, day, month persistence worker -> storage
after: metrics entrance -> aggregate worker -> remote worker -> minute persistence worker ->  trans worker -> hour, day, month persistence worker -> storage

* First commit

* Fix e2e test and some nits

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

* Feature of database session

* Make it configurable.

* Make the OAP server can't startup.

* Update demo environment

* Update Jenkinsfile

* servlet 2.5

* method exist

* update MethodUtil

* Update MethodUtil.java

Add a comment to the new core util method. @candyleer I am concerning this method will be used widely even unnecessary.

fixed for #3135:createEntrySpan should get SamplingService only if it required ( carrier is valid) (#3137)

* Refactor Persistence worker.

* 1. Provide InsertRequest and UpdateRequest interface for prepare persistence.
2. Implement the ids query for H2 metrics DAO.

* Refactor worker framework

* Use queue to receive asynchronous batch request.

* Rename the Datacarrier thread name.

* Fixed some mistake.

* New mistake.

Make agent mock works as usual.

2. Implement the ids query for H2 metrics DAO.

* Make use of multi-thread builds of maven

* Bump up maven version to get rid of parallel build bug

* Apply the same to E2E tests

* Apply the same to JavaDoc task

* Revert E2E settings

* Add maven tuning opts

* Increase maven fork process memory

* Remove the worker id, and add worker name for remote handler only.

* Remote metrics and inventory classes mapping too.

* Refactor codes.

* Improve OAP server performance.

* Fix 3096

* Override ES settings in FAQ

* Resend traffic data when invalid

* Remove unsupported ES setting

* Use seperated buffer directory and remove unnecessary web app 2

* Fixed the problem of dirty reading of the storage implementation of elasticsearch.

* Fixed a mistake.

* Make finish more stable in async APIs

* Change OAL generator tool to runtime.

Step 1. change project structure. API links and maven pom.

* Part of metrics class generation

* Metrcis class generated.

* Set up the basic structure of new OAL engine.

* Finish metrics generation.

* Support dispatcher generation.

* Format codes.

* Generate dispatcher all methods.

* Implement disable in hardcode.

* Clear up

* Fix compile startup.

* Update license and document of new OAL engine.

* Fix rat.

* Disable class output in default

* Copy from @kezhenxu94 's test PR, https://github.com/apache/skywalking/pull/3110

* Wait for the mount command to finish

* improve ContextManager.stopSpan and awaitFinishAsync performance: call ThreadLocal only once
For the [JMH benchmark test](https://gitlab.com/qxo1/Common-JMH-benchmark/-/jobs/251025992) :
On gitlab CI(java 8): call ThreadLocal once vs twice fast over 10%
My T480 java 8 : fast over 40%; java 11: fast over 50%;

[The JMH benchmark code ](https://github.com/qxo/Common-JMH-benchmark/blob/master/src/main/java/test/ThreadLocalGetXTimeBenchmark.java)

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* fix 3069

* Using ArrayList to instead of LinkedList

* Add docs of e2e development guide

The e2e containers' names of single mode and cluster mode
would conflict when they start up simultaneously in Jenkins job.

* Correct spelling errors in the network protocol.

* update protocol submodule commit id.

* Fix wrong logger name

* Fix more wrong logger names in agent side

* Add e2e cluster tests

* setup your idea

* add anchors

* fix

* Update docs/en/guides/How-to-build.md
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>

* Update docs/en/guides/How-to-build.md
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>