feat: add prometheus metrics support

[Niahh]

This PR introduces Prometheus-compatible metrics to the NSSF to support observability and performance monitoring in Free5GC deployments.

Key Features

• SBI Metrics: Track inbound/outbound HTTP requests on the Service-Based Interface (SBI) with counters and latency histograms.
• Built-in Metric Server: Exposes a /metrics HTTP endpoint that is configurable and compatible with Prometheus scraping.
• Modular Design: Metrics logic is isolated in a dedicated metrics package in the Util project to minimize code coupling and simplify reuse across NFs.
• No External Dependencies: Uses the official Prometheus Go client library only — no sidecars or exporters required.

Motivation

Introducing native metrics is a foundational step toward:

• Understanding real-time behavior of the NSSF
• Coupled with testing tools such as PacketRusher, it can help with debugging some scenarios.
• Integrating Free5GC into production-grade observability stacks (e.g., Prometheus + Grafana)

Configuration

The metrics server is disabled by default. To enable, add the proper section :

# Metrics configuration
# If using the same bindingIPv4 as the sbi server, make sure that the ports are different
metrics:
	enable: true # (Optional, default false)
	scheme: http # (Required) the protocol for metrics (http or https, default https)
	bindingIPv4: 127.0.0.18 # (Required) IP used to bind the metrics endpoint (default 0.0.0.0)
	port: 9091 # (Optional, default 9091) port used to bind the service
	tls: # (Optional) the local path of TLS key (Could be the same as the sbi ones)
		pem: /home/sam/private-network/code/free5gc/cert/NSSF.pem # NSSF TLS Certificate
		key: /home/sam/private-network/code/free5gc/cert/NSSF.key # NSSF TLS Private key
	namespace: free5gc # (Optional, default free5gc)

Backward Compatibility

100% backward compatible – metrics support is optional and deactivated until activated by the user.

Integration

The same metrics architecture is being adapted across other Free5GC NFs (AUSF, NRF, SMF , etc.) to ensure consistency and maintainability.

Pictures

Example of metrics produced

This work is sponsored by Free Mobile!

[leowu0407]

@ianchen0119 @Alonza0314 LGTM

[Alonza0314]

LGTM