func Register(b Builder) {
	m[strings.ToLower(b.Name())] = b
}
Register registers the balancer builder to the balancer map. b.Name
(lowercased) will be used as the name registered with this builder.  If the
Builder implements ConfigParser, ParseConfig will be called when new service
configs are received by the resolver, and the result will be provided to the
Balancer in UpdateClientConnState.

NOTE: this function must only be called during initialization time (i.e. in
an init() function), and is not thread-safe. If multiple Balancers are
registered with the same name, the one registered last will take effect.
package balancer
import (
	"context"
	"encoding/json"
	"errors"
	"net"
	"strings"
	"google.golang.org/grpc/connectivity"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/internal"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/resolver"
	"google.golang.org/grpc/serviceconfig"
)
var (
	m = make(map[string]Builder)
)