Ensure that gRPC metrics headers are in the order requested by the spec.

The present order in `api.core` is:

```
core/0.27.1 gl-python/3.6.0 grpc/0.27.1 gapic/0.27.2
```

The order per spec should be:

```
gl-python/2.7.12 gccl/0.21.0 gapic/0.1.0 core/0.14.1 grpc/1.0.1
```