feat: add prometheus metrics support

[Niahh]

This PR introduces Prometheus-compatible metrics to the NRF 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 NRF
• 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/NRF.pem # NRF TLS Certificate
		key: /home/sam/private-network/code/free5gc/cert/NRF.key # NRF 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, AMF, SMF , etc.) to ensure consistency and maintainability.

This work is sponsored by Free Mobile!

[Niahh]

Working on aliging the PR with all the errors/warning from golangci-lint and taking the latest change of upgrad_cli_to_v2

[senselessDog]

@Alonza0314 @ianchen0119 This PR can be merged.

[Alonza0314]

LGTM

lint error

[ianchen0119]

@Niahh

Would you please help fix the linter error?

Thank you!

[Alonza0314]

The check errors also exist at tngf and udm.

[Niahh]

@Niahh

Would you please help fix the linter error?

Thank you!

Hi @ianchen0119, @Alonza0314

Sure thing ! Sorry I missed the nrf in my round of check for the lint errors !