Begin to provide telemetry and observability for SkyWalking own backend

[吴晟 Sheng Wu <wu...@foxmail.com>]

Hi


SkyWalking 6 is closed to GA. More and more companies have used the SkyWalking in production environment. 
I want to make sure SkyWallking cluster itself is stable enough, and also provides enough observability to ops team.


So, I propose this new feature[1]. As first step, I provide APIs and Noop implementor in this pull request[2].
The following steps are
1. Provide Prometheus telemetry provider
2. Design and add metric to the whole backend core.


Feedback welcome.


[1] https://github.com/apache/incubator-skywalking/issues/2073
[2] https://github.com/apache/incubator-skywalking/pull/2123


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Sheng Wu
Apache SkyWalking, ShardingSphere, Zipkin
Twitter, wusheng1108