Here are the list of changes we made in this pull request. Top 3 are the principle changes, others are following these principles.
1. New agent and mesh report protocol. 
2. New agent header protocol. 
3. Service register, instance register and network address register have been removed permanently.
4. Service traffic, instance traffic and network alias metrics are added to replace the service, instance and network address inventory.
5. Register process has been removed.
6. Metrics stream process supports insert only mode, especially for traffic entities.
7. Metrics stream process supports no-downsampling mode for traffic entities and network alias.
8. Remove all register mechanism and cache in the java agent.
9. Remove MONTH step in GraphQL query.
10. Update UI to remove MONTH step query, the max query range is 60 days now.
11. Simplify the TTL to metrics and record. And the unit has been formatted in Day unit. No specific TTL for ElasticSearch storage.
12. Buffer mechanism of trace receiver and mesh receiver has been removed due to no register.
13. New service id, instance id and endpoint id rules, including service relation, instance relation and endpoint relation id rules.
14. Java agent support `keep tracing` mode, meaning, agent generating tracing context even the backend is unconnected/unavailable.
15. Plugin test tool up to date, in order to support new protocol.
16. Plugin tests expected data files updated.
17. E2E tests updated.
18. Telemetry of Grafana config has been merged into one.
19. Documentation updates.
20. [TBD] InfluxDB storage implementation is not available, need @dmsolr to fix later, in order to reduce the master change block by this PR.

* support http api.

* e2e
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>
Co-authored-by: Nkezhenxu94 <kezhenxu94@apache.org>

* Support day/hour/minute metrics merging into one index. Reduce the number of index 50%.

Motivation:

Review code styles with some bots automatically.

Modifications:

Set up ReviewDog in GitHub Action to review code style.

Add more check rules to checkstyle plugin.

Result:

Obvious code styles can be reviewed and commented automatically.

* Drop support of protocol v1 (sw3)

* Remove versioning logic further

* Remove more version5-only classes

* Commit missing files

* Update submodule and address review comment

* Fix failed tests

* Update JDK compiling level to 8, and remove zip dist.

* Exclude annotation and shade API.

* Fix more exclusion.

* 1. Update grava and gson 2. Fix compile issue. 3. Keep protoc and maven plugin in old version. Some incompatible of 3rd party grpc proto.

* Fix dependency conflicts, license and notice issue, and dependency check script issue
Co-authored-by: Nkezhenxu94 <kezhenxu94@163.com>

* Remove the local span and exit span register mechanism in Java agent scenario.

* Fix config, plugin, and document, as exit span is not required register anymore.

* Try to fix the test case expected data due to no register for exit span.

* Fix toolkit case.

* Remove exit span from plugin test framework document.

* Update protocol, FAQ, and CHANGELOG documents to highlight this change.

* Fix NPE when no entry span in the whole segment. Set endpoint to null in segment entity.

* Fix the snapshot and add inexistence operation name id.

* Fix several async cases.

* Fix undertow test case.

* Fix wrong error log name.

* Keep name consistent with 6.x concepts.

* Try to fix 3 async scenarios.

* Fix okhttp case.

* Fix Spring Async case.

* Fix another NPE

* Fix NPE when first span has no endpoint id when it is local or exit span.

* Still fill first operation name when no endpoint id.

Make agent mock works as usual.

Get the document by index name.

* The prototype of topN sql worker.

* Add scope and new manual dispatcher.

* no message

* Finish topN persistence codes. Not test yet. And query have not added.

* Finish the top n database statement persistent.

* Support different slow thresholds for different db types.

* Fix wrong db default threshold

* Finish new query protocol binding.

* Provide query empty implementation and sync ui.

* Finish all codes, hope it works :P
I will run the tests tonight.

* Fix

* Fix

* Fix a startup issue.

* Add time bucket to persistent.

* Fix wrong query result column name.

* Fix Database query.

* Fix checkstyle.

* Fix wrong order logic, and add a test case to verify, to fix https://github.com/apache/incubator-skywalking/pull/2239#discussion_r255948249

* Support sampling trace at server side and keep metric right.

* Add a trace sampling document

* Fix wrong default value.

* Fix document issue.

* Fix assemble issue.

* Fix wrong settings and doc.

Also fixed several bugs in this merge.

2. Close the input stream when switch to read next file.

#1665

2. Fixed the bug of agent mock register order.
3. Fixed the bug of all heatmap.

2. Make service relation and endpoint relation to record component id.
3. Delete service mapping indicator.
4. Make service inventory to record mapping service id.
5. Refactor global topology query and service topology query.

* Fixed the bug from topology.

* no message

* Add generate indicator and dispatcher about service topology but not test.

* Delete call type from service relation.

* Streaming analysis topology test success.

* Fixed the check style error.