Metrics Registration Fails Due to Empty `allMetrics` Slice

[Yashas-naidu]

Repository: [IBM/Portieris]

Issue Description:
In the NewMetrics function of metrics.go, the call to prometheus.MustRegister(p.allMetrics...) happens before any metrics are added to allMetrics. Since allMetrics is initially empty, this can cause a panic or unexpected behavior.

Steps to Reproduce:

1. Instantiate PortierisMetrics using NewMetrics().
2. Observe a runtime error if allMetrics is empty at registration time.

Proposed Fix:
Register each metric immediately upon creation in the counter method instead of registering them in NewMetrics().

func (p *PortierisMetrics) counter(name, help string) prometheus.Counter {
	result := prometheus.NewCounter(prometheus.CounterOpts{
		Name: metricName(name),
		Help: metricHelp(help),
	})

	prometheus.MustRegister(result) // Register the metric immediately
	p.allMetrics = append(p.allMetrics, result)
	return result
}

Expected Behavior After Fix:

• Each metric gets registered as soon as it is created, avoiding any issues with an empty allMetrics slice.
• NewMetrics() initializes correctly without errors.

Additional Context:
This issue aligns with Prometheus best practices, ensuring that metrics are registered as they are defined rather than in bulk.