documentation: on server, use FromIncomingContext for retrieving context and SetHeader, SetTrailer to send metadata to client

[purnesh42H]

Fixes: #7215. It was opened while addressing the question #7125 in order to make it more clear for sending metadata from server to client. The sending metadata documentation section specifies it clearly in grpc-metadata.md to use grpc.SetTrailer

However, we should make it more explicit to use FromIncomingContext on server as original issue was using OutgoingContext in server interceptor. Moreover, we should replace SendHeader to SetHeader in documentation as SendHeader is used to send the headers immediately to the client where as SetHeader is used to set headers that will be sent with the gRPC response, which is a more common use case.

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.45%. Comparing base (adf976b) to head (0e6496c).
Report is 85 commits behind head on master.

❗ Current head 0e6496c differs from pull request most recent head a13ca0c

Please upload reports for the commit a13ca0c to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7238      +/-   ##
==========================================
- Coverage   81.24%   80.45%   -0.79%     
==========================================
  Files         345      348       +3     
  Lines       33941    33998      +57     
==========================================
- Hits        27574    27354     -220     
- Misses       5202     5449     +247     
- Partials     1165     1195      +30     

see 48 files with indirect coverage changes

[easwars]

The else block can be eliminated and the block of code within it can be moved outside of the if block.

	header, err := c.Header()
	if err != nil {
		log.Fatalf("Receiving headers: %v", err)
	}
	
	fmt.Println("Received headers:")
	for k, v := range header {
		fmt.Printf("%s: %v\n", k, v)
	}

[purnesh42H]

Good catch. I forgot to remove it

[easwars]

Go style does not impose line length restrictions on code lines, and it is recommended to not break-up code lines unless it significantly improves readability. See: https://google.github.io/styleguide/go/guide#line-length

You could simply write this as:

	resp, err := client.UnaryEcho(ctx, &pb.EchoRequest{Message: "hello world"}, grpc.Header(&header), grpc.Trailer(&trailer))
	if err != nil {
		log.Fatalf("UnaryEcho: %v", err)
	}

Or if you don't like such long lines, you could do:

	opts := []grpc.CallOption{grpc.Header(&header), grpc.Trailer(&trailer))}
	resp, err := client.UnaryEcho(ctx, &pb.EchoRequest{Message: "hello world"}, opts...)
	if err != nil {
		log.Fatalf("UnaryEcho: %v", err)
	}

[purnesh42H]

Done

[easwars]

Comments that are complete sentences should be capitalized and punctuated like standard English sentences. Here and elsewhere in this PR.

See: https://google.github.io/styleguide/go/decisions#comment-sentences

[purnesh42H]

Done