[bug] backendnum statistics error in radon slave

[zhya]

The slave node of Radon will synchronize metadata from the master node. At this time, backend.json will be reloaded, causing backendnum to be counted repeatedly.

backend_number{instance="cln-h7eywl6v",job="radon",type="backend"}  46
backend_number{instance="cln-j6ktmwdo",job="radon",type="backend"} 2

We need set 0 before reload the backend config.