Make the proxy to report the number of clients to the broker

So the assignment of proxies is based on the load. The number of clients
is ronded down to 8. Existing proxies that doesn't report the number
of clients will be distributed equaly to new proxies until they get 8
clients, that is okish as the existing proxies do have a maximum
capacity of 10.

Fixes #40048

common/messages/proxy.go

@@ -17,8 +17,9 @@ const version = "1.2"
 {
   Sid: [generated session id of proxy],
   Version: 1.2,
-  Type: ["badge"|"webext"|"standalone"]
-  NAT: ["unknown"|"restricted"|"unrestricted"]
+  Type: ["badge"|"webext"|"standalone"],
+  NAT: ["unknown"|"restricted"|"unrestricted"],
+  Clients: [number of current clients, rounded down to multiples of 8]
 }
 
 == ProxyPollResponse ==
@@ -79,43 +80,48 @@ type ProxyPollRequest struct {
 	Version string
 	Type    string
 	NAT     string
+	Clients int
 }
 
-func EncodePollRequest(sid string, proxyType string, natType string) ([]byte, error) {
+func EncodePollRequest(sid string, proxyType string, natType string, clients int) ([]byte, error) {
 	return json.Marshal(ProxyPollRequest{
 		Sid:     sid,
 		Version: version,
 		Type:    proxyType,
 		NAT:     natType,
+		Clients: clients,
 	})
 }
 
 // Decodes a poll message from a snowflake proxy and returns the
-// sid and proxy type of the proxy on success and an error if it failed
-func DecodePollRequest(data []byte) (string, string, string, error) {
+// sid, proxy type, nat type and clients of the proxy on success
+// and an error if it failed
+func DecodePollRequest(data []byte) (sid string, proxyType string, natType string, clients int, err error) {
 	var message ProxyPollRequest
 
-	err := json.Unmarshal(data, &message)
+	err = json.Unmarshal(data, &message)
 	if err != nil {
-		return "", "", "", err
+		return
 	}
 
 	majorVersion := strings.Split(message.Version, ".")[0]
 	if majorVersion != "1" {
-		return "", "", "", fmt.Errorf("using unknown version")
+		err = fmt.Errorf("using unknown version")
+		return
 	}
 
 	// Version 1.x requires an Sid
 	if message.Sid == "" {
-		return "", "", "", fmt.Errorf("no supplied session id")
+		err = fmt.Errorf("no supplied session id")
+		return
 	}
 
-	natType := message.NAT
+	natType = message.NAT
 	if natType == "" {
 		natType = "unknown"
 	}
 
-	return message.Sid, message.Type, natType, nil
+	return message.Sid, message.Type, natType, message.Clients, nil
 }
 
 type ProxyPollResponse struct {