client go-generator should allow adding custom headers to outgoing requests

[evantorrie]

Currently, the generator for client.go writes code that initializes outgoing HTTP headers to an empty map, and adds only what it needs internally for its specific purposes (e.g. Credentials header/value and If-None-Match ETag type headers)

It would be good to allow custom headers to be added to each client on a per-request basis.

[boynton]

This would be a worthwhile enhancement.

[boynton]

Do we want both custom "sticky" headers like the current credentials, as well as per-request custom headers? The latter would most easily be done by modifying the generated API to take an extra argument (a http.Header), which would create incompatibilities. The sticky headers would be trivial to add, in comparison

[evantorrie]

In the case I'm thinking about, we would be ok with sticky credentials - so long as there was some way to "update" them (e.g. to refresh a header value). Even if that meant calling "AddHeaders()" again, that would probably be ok.

[boynton]

Would probably need to add API to SetHeader instead of AddHeader (which will accumulate a list of values). Sticky headers is easy enough that I can see doing that now, per-request headers may require some API work, not quite ready to do that.

[gotwarlost]

Why couldn't you do all this in a custom transport? We already do user-agent injection and other stuff like this using a custom transport.

[boynton]

Good point. The other reason I've dragged my feet is that headers are technically part of the API, and I'd prefer API details to be explicit in the RDL itself (an approach which also allows adding whatever headers you want in a more formal way).