test(metrics): add unit tests for getMetricsRefreshInterval

[Neha-Nimsatkar]

test(metrics): add unit tests for getMetricsRefreshInterval

This PR adds unit tests for getMetricsRefreshInterval() in the backend/pkg/metrics package. Currently, the metrics package does not have test coverage for environment variable parsing.

The tests cover all parsing scenarios and fallback behaviors:

• Default fallback when NEBRASKA_METRICS_UPDATE_INTERVAL is unset
• Valid custom intervals in seconds and minutes (e.g., 30s, 2m)
• Fallback handling when invalid duration strings, negative durations, or zero durations are passed

How to use

Reviewers can validate this change by running the Go test suite inside the backend package.

Testing done

• Formatted code using go fmt ./pkg/metrics/...
• Ran the unit test suite locally:

cd backend
go test -v ./pkg/metrics/...