Support distributed tracing between user-application with tidb and other componentes

## Feature Request

**Is your feature request related to a problem? Please describe:**
When we enabled opentracing both in our application (e.g.: a java web) and `tidb`, what we finally get is 2 seperated tracing results: one is application tracing and the other is `tidb` inside tracing.

**Describe the feature you'd like:**
what need to be done first:
1. enabled `opentracing` in `tidb`.
2. use `opentracing`/`opentelemtry` instrumentation in `applications`

what we expected:
A full tracing result cover the application and `tidb` both. Additionally, it it contains `pd-server` and `tikv` would be perfect.

**Describe alternatives you've considered:**
1. Need a way to pass some trace infos from `app` to `tidb` (like `trace-id`)
2. `tidb` **client** which request to `pd-server` and `tikv` need instrumentation by tracing sdk.
3. `pd-server` and `tikv` need instrumentation by tracing sdk.

**Teachability, Documentation, Adoption, Migration Strategy:**
If this can be done, when the `application service` cost too much time in some request , we can find out the real cost in the backend service and connect all component in one request. That would be a convenient way for optimize and error-tracking.

By the way, I can only find `opentracing` supported in `tidb-server` til now, if there're anything I've missed plz let me know, thanks very much!