Grpc timeout not set for warmup requests

[bjornm]

Describe the bug

We run all our grpc servers with deadline enforcement. Requests which don't have a deadline (the grpc-timeout header) will be rejected. However, mittens does not propagate the deadline for warmup requests, only for the initial reflection request.

To Reproduce

Send grpc requests to a server and debug the incoming headers. You will notice that the grpc-timeout header is missing from the warmup requests.

Expected behavior

grpc-timeout should be set for all grpc requests.

Additional context

It is not possible to add the header manually, as the "grpc-timeout" header is reserved by the golang grpc implementation and will be removed prior to sending, if you try to supply it manually.

Will send a PR fix shortly.