OpenTelemetry Support

## Feature Request

**Is your feature request related to a problem? Please describe:**

We're trying to trace requests through TiDB, but our tooling requires OpenTelemetry, and TiDB uses OpenTracing.

**Describe the feature you'd like:**

Ideally we'd be able to easily trace requests through TiDB using OpenTelemetry.

**Describe alternatives you've considered:**

We could try to adapt OpenTracing to OTEL Tracing in our collector, but that wouldn't solve it for everyone, just us, and we might have to change things if it gets added "officially" in the future.

**Teachability, Documentation, Adoption, Migration Strategy:**

OpenTelemetry maintains a migration doc ( https://opentelemetry.io/docs/migration/opentracing/ ). I imagine the steps are:

- add OTEL traces in addition to OpenTracing traces (where both are behind a config flag)
- Update any tooling to use OTEL traces
- Deprecate OpenTracing
- Remove OpenTracing