pass headers to reflection client

[dbarrosop]

Fixes #347

So we just run into this problem. We have several services behind traefik and we distinguish between them by their service/package. However, because reflection runs on its own service/package namespace this doesn't work as there is no way of telling which service it's supposed to be querying to. To solve that we added a header to redirect to one service or another but evans doesn't pass the specified header to the reflection clients.

This PR takes the headers specified via the command line and passes them to the reflection clients, it doesn't introduce any new flag or modify any other behavior.

Not sure if this is how you wanted to solve this problem or if someone is working on this already but as I needed something quick I figured I'd fixed and open a PR to kick the discussion.

thanks

[codecov]

Codecov Report

Merging #351 (ab5c40e) into master (62389c7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   80.29%   80.31%   +0.01%     
==========================================
  Files          57       57              
  Lines        2736     2738       +2     
==========================================
+ Hits         2197     2199       +2     
  Misses        309      309              
  Partials      230      230              

[ktr0731]

Thanks for submitting the PR.
Most of the changes, and the policy of not changing the command interface, are completely in line with my assumptions!
I suggested a nitpick change, so could you take a look?

[ktr0731]

Suggested change

	md := metadata.New(nil)
	for k, v := range headers {
	md.Append(k, v...)
	}
	return metadata.NewOutgoingContext(context.Background(), md)
	return metadata.NewOutgoingContext(context.Background(), metadata.MD(headers))

[dbarrosop]

Thanks for the review, I addressed the comment and squashed the commits. Let me know if there is something else you need.

[ktr0731]

LGTM, thanks for your contribution!