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

[theacodes]

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

[lukesneeringer]

And core should be spelled gax.

[theacodes]

Related: https://github.com/grpc/grpc/pull/12778/files

[theacodes]

Part of me kind of wants to sit on this until intercepters are ready.