F5 BIG-IP Irule - redirect to individual node for external monitoring

Change the second part(replace) of your irule like this :

when HTTP_REQUEST { 
 Check if path contains /monitoring 
    if {[HTTP::path] contains "/monitoring/"}{ 

 use getfield to split the uri based on monitoring/ and then return the server name after that
    set server_id [getfield [HTTP::uri] "monitoring/" 2 ]
   log local0. "Server id: $server_id"

 Replace /monitoring and servername in the path 
    set uri [string range [HTTP::uri] 0 [expr {[string first "/monitoring/" [HTTP::uri]] - 1}]] 
    log local0. "orginal part of uri: $uri"
    HTTP::uri $uri

redirect or rewrite uri?
  HTTP::redirect "http://[HTTP::host]$uri"
    pool "/Common/${server_id}"
    } 
}

If I were you, I would use node ip (and w/o node port) instead of Server name, like this:

when HTTP_REQUEST { 
 Check if path contains /monitoring 
    if {[HTTP::path] contains "/monitoring/"}{ 

 use getfield to split the uri based on monitoring/ and then return the node ip after that
    set server_id [getfield [HTTP::uri] "monitoring/" 2 ]
   log local0. "Node IP: $node_ip"

 Omit /monitoring and nodeip in the path 
    set uri [string range [HTTP::uri] 0 [expr {[string first "/monitoring/" [HTTP::uri]] - 1}]] 
    log local0. "orginal part of uri: $uri"
    HTTP::uri $uri

 Send the request to the specified node
    node $node_ip 80
    
    } 
}