So that you inform reverse proxy servers to use it for routing.
Tested with nginx/1.13.12, which is the latest release of nginx as of today.
Usage: `grpcurl --authority example.com localhost`
See https://github.com/grpc/grpc-go/pull/1064 for the PR to grpc-go which enabled this feature.
See https://tools.ietf.org/html/rfc7540#section-8.1.2.3 and https://tools.ietf.org/html/rfc3986#section-3.2 for what is a valid :authority header value.
Note that `grpcurl -H ':authority: example.com' localhost` doesn't work in two ways.
One is that the current implementation of grpcurl splits `:authority: example.com` to `[]string{"", "authority: example}` and considers it as a header with an empty key and a value of "authority: example.com" which isn't valid.
Another one is that `-H` passes request metadata, whereas `:authority` header must be provided at connection time.
* add more control over request metadata between reflection calls and main RPC invocation
* add flag to print a message template (when describing message types)
* relax some command-line argument issues to warnings
Yusuke KUOKA