MetricsPort not working for softmaps

Description

The tor_metrics_port configuration right now is only working for hardmap, as softmap uses many tor daemon instances causing a conflict when trying to open the same metrics port.

Tasks

• Implement MetricsPort support for many backends within the same project (softmap case).
• Update the documentation explaining how MetricsPort is set for softmap.

Time estimation

• Complexity: small (1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)