server: set CLIENT_CONNECT_ATTRS

[dveeden]

package main

import (
	"net"

	"github.com/go-mysql-org/go-mysql/server"
)

func main() {
	l, _ := net.Listen("tcp", "127.0.0.1:4000")
	c, _ := l.Accept()
	conn, _ := server.NewConn(c, "root", "", server.EmptyHandler{})
	for akey, aval := range conn.Attributes() {
		println(akey, aval)
	}
}

go run main.go

mysql -h 127.0.0.1 -P 4000 -u root 

With this PR there is output, without this PR there is not (unless one would explicitly set CLIENT_CONNECT_ATTRS)

_client_name libmysql
os_user dvaneeden
_client_version 9.2.0
program_name mysql
_pid 470697
_platform x86_64
_os Linux

[dveeden]

Compatibility testing:

mycli 1.27.2

dvaneeden@dve-carbon:/tmp/attrs$ go run main.go 
_client_name pymysql
_client_version 1.1.1
_pid 472593
program_name mycli

MySQL 5.7.31

dvaneeden@dve-carbon:/tmp/attrs$ go run main.go 
_platform x86_64
program_name mysql
_os linux-glibc2.12
_client_name libmysql
_pid 473779
_client_version 5.7.31

MySQL 5.6.44

dvaneeden@dve-carbon:/tmp/attrs$ go run main.go 
_client_name libmysql
_pid 473985
_client_version 5.6.44
_platform x86_64
program_name mysql
_os linux-glibc2.12

MySQL 5.5.62 (not sending anything)

dvaneeden@dve-carbon:/tmp/attrs$ go run main.go