Add benchmark matrix for connect protocols

[emcfarlane]

Previous benchmarks only tested gRPC clients. This extends the matrix for all protocols and each streaming type. Streams do a single req/rsp to make it easier to compare to unary flows.

go test -run=^$ -bench=BenchmarkConnect
goos: darwin                                        
goarch: arm64                                                     
pkg: connectrpc.com/connect                                      
BenchmarkConnect/connect/unary_big-8                 517           1978663 ns/op         5784987 B/op        171 allocs/op
BenchmarkConnect/connect/unary_small-8             29280             40443 ns/op           21701 B/op        165 allocs/op
BenchmarkConnect/connect/client_stream-8           20979             57211 ns/op           22458 B/op        206 allocs/op
BenchmarkConnect/connect/server_stream-8           20085             56513 ns/op           22721 B/op        211 allocs/op
BenchmarkConnect/connect/bidi_stream-8             20209             58290 ns/op           21546 B/op        188 allocs/op
BenchmarkConnect/grpc/unary_big-8                    537           2037258 ns/op         5940546 B/op        231 allocs/op
BenchmarkConnect/grpc/unary_small-8                25891             46011 ns/op           24805 B/op        223 allocs/op
BenchmarkConnect/grpc/client_stream-8              25942             45948 ns/op           26083 B/op        228 allocs/op
BenchmarkConnect/grpc/server_stream-8              26628             45404 ns/op           26277 B/op        234 allocs/op
BenchmarkConnect/grpc/bidi_stream-8                25088             48019 ns/op           21529 B/op        211 allocs/op
BenchmarkConnect/grpcweb/unary_big-8                 541           2059821 ns/op         6236843 B/op        218 allocs/op
BenchmarkConnect/grpcweb/unary_small-8             20353             57924 ns/op           30645 B/op        211 allocs/op
BenchmarkConnect/grpcweb/client_stream-8           20475             59153 ns/op           30904 B/op        216 allocs/op
BenchmarkConnect/grpcweb/server_stream-8           20637             57831 ns/op           31755 B/op        221 allocs/op
BenchmarkConnect/grpcweb/bidi_stream-8             20388             58601 ns/op           20090 B/op        189 allocs/op          
PASS                                                              
ok      connectrpc.com/connect  24.837s   

[akshayjshah]

Not super surprised, but it's nice to see that the Connect protocol is the most efficient unary implementation.

[emcfarlane]

@akshayjshah let me know what else is needed for this PR. Would be great to have for testing the changes to unary flows!

[akshayjshah]

LGTM! I wasn't sure about expanding the example PingServer implementation, but it's probably good to show how to implement all the different RPC types in the GoDoc.

[akshayjshah]

Updated the commit description before merge. For posterity:

$ BENCH=BenchmarkConnect make bench

BenchmarkConnect/connect/unary_big-12         	   1041	  1183182 ns/op	6379285 B/op	    175 allocs/op
BenchmarkConnect/connect/unary_small-12       	  25263	    46785 ns/op	  19597 B/op	    151 allocs/op
BenchmarkConnect/connect/client_stream-12     	  17299	    69526 ns/op	  26505 B/op	    189 allocs/op
BenchmarkConnect/connect/server_stream-12     	  17530	    69564 ns/op	  26562 B/op	    184 allocs/op
BenchmarkConnect/connect/bidi_stream-12       	  17264	    68655 ns/op	  22356 B/op	    172 allocs/op
BenchmarkConnect/grpc/unary_big-12            	    867	  1193301 ns/op	6283560 B/op	    234 allocs/op
BenchmarkConnect/grpc/unary_small-12          	  20544	    57809 ns/op	  23892 B/op	    209 allocs/op
BenchmarkConnect/grpc/client_stream-12        	  19783	    59283 ns/op	  24501 B/op	    212 allocs/op
BenchmarkConnect/grpc/server_stream-12        	  20704	    57771 ns/op	  24781 B/op	    206 allocs/op
BenchmarkConnect/grpc/bidi_stream-12          	  19772	    57777 ns/op	  20032 B/op	    195 allocs/op
BenchmarkConnect/grpcweb/unary_big-12         	    859	  1215487 ns/op	6008795 B/op	    220 allocs/op
BenchmarkConnect/grpcweb/unary_small-12       	  17203	    68704 ns/op	  30543 B/op	    196 allocs/op
BenchmarkConnect/grpcweb/client_stream-12     	  17422	    68824 ns/op	  30042 B/op	    199 allocs/op
BenchmarkConnect/grpcweb/server_stream-12     	  17274	    68969 ns/op	  30180 B/op	    193 allocs/op
BenchmarkConnect/grpcweb/bidi_stream-12       	  15910	    74695 ns/op	  21374 B/op	    172 allocs/op