feat: measure each direction of the speed test for 7 s (0.0.7)

internal/proto/proto.go

@@ -184,6 +184,9 @@ type SysSpeedArgs struct {
 	// Relay makes the agent use the relayed streams, which the server pipes
 	// to and from the browser.
 	Relay bool `json:"relay,omitempty"`
+	// WindowMs is how long to measure each direction for; 0 leaves it to the
+	// agent (3 s, all an agent older than 0.0.7 does).
+	WindowMs int `json:"window_ms,omitempty"`
 }
 
 // SysSpeedResult is returned for TaskSysSpeed: the agent's HTTP link to the