Parent spans at the middleware using OpenTelemetry

## Bug Report

### Version
└── tonic v0.6.2
└── tonic-build v0.6.2
├── tonic v0.6.2 (*)

### Platform

MINGW64_NT-10.0-19043 3.1.7-340.x86_64 2021-03-26 22:17 UTC x86_64 Msys

### Description

I tried to create spans using `OpenTelemetry`, `tonic`, and `tower-http`.
However, I couldn't create a parent span at the middleware.

I prepared my simple [example](https://github.com/arttet/simple-service).


I expected to see one span.

P.S. Maybe I did something wrong.