Start Burp Suite and ensure the proxy listener is running on 127.0.0.1:8080.
Launch Metasploit:
msfconsole
Select the desired HTTP module:
use auxiliary/scanner/http/robots_txt
Configure Burp as the proxy:
set PROXIES HTTP:127.0.0.1:8080
Set the target:
set RHOST <TARGET_IP>
set RPORT <PORT>
Execute the module:
run