Avoid nil point when config.Metrics is nil and expose all metrics if none are configured

[TingluoHuang]

When metrics are configured on the controller, but no metrics are applied to the scale set, the listener would panic with nil dereference error.

This change applies all metrics by default if metrics are configured, but no customisation is being done.

To avoid server errors in the future, defaults are applied to the server address and the default endpoint. This code path should not be hit by default, since the app.go decides if it should apply metrics based on the server address. But it made sense to use the default :8080 port and /metrics endpoint in the default function to future-proof the code.

Fixes #3993

[Copilot]

Pull Request Overview

This PR prevents a nil pointer dereference when config.Metrics is nil by separating the exporter configuration creation and only assigning Metrics when it’s non-nil.

• Extracted metrics.ExporterConfig into a local variable.
• Added a nil check around config.Metrics before assignment.
• Moved NewExporter call to use the pre-built config.