Skywalking: Contribute `baseline` computing module from jd.com

[wu-sheng]

We are glad to accept your contributions. As we told, this baseline computing module is from you private/internal APM system, this will help skywalking to provide more features. Thanks.

Here is my guideline:

1. The whole collector is based on modulization core. So I define a module for your team, named baseline-computing, and path is /apm-collector/apm-collector-baseline/collector-baseline-computing-define. You should define the baseline services, which represent as Java interfaces, like many rpc frameworks did. You should register them in module define through the return list of ComputingModule#services.

    @Override public Class[] services() {
        return new Class[0];
    }

1. You need to provide a provider for this module, that is why I set up a provider, named default, and path is /apm-collector/apm-collector-baseline/collector-baseline-computing-provider.
2. Bind the services and implementations in #prepare, by using #registerServiceImplementation. The serviceType parameter should be the service, you defined in module, and the service parameter should be your implementation of the first parameter service.
3. Set the test cases about how to use the your services, and make sure the test coverage is over 80% in provider module.

And there are some attentions:

1. check-sytle works, so make sure you imported the style file, and ci passed.
2. License head is in java.header file, please make sure your IntelliJ preference setting is right. It should add the heads automatically.
3. Make sure @ustbsjl and @beforeeight set the github right.
4. Link your pull request to this issue.

Looking forward your pull request.

If you need any help, you know how to find me.

Thanks.
Skywalking PMC team.

[wu-sheng]

FYI @OpenSkywalking/pmc @OpenSkywalking/committer-team @OpenSkywalking/huawei-devcloud-contributor-team

[peng-yongsheng]

I care about the data structure of the services.

[peng-yongsheng]

@beforeeight
Can you share some experience about the baseline?

1. Which metrics need to do baseline calculate.(application/instance/service 's error/response time/rate.... anything else?)
2. Which time duration? (every ? minute/hour/day)
3. How often need to do baseline recalculate.(The number of services are so huge. )