add /metrics endpoint to backstage framework

Need to add monitoring endpoint to backstage. Currently, it is not enabled by default in the base framework. Need to enable this to allow monitoring for backstage.

values.yaml blob: https://repo1.dso.mil/big-bang/product/packages/backstage/-/blob/main/chart/values.yaml?ref_type=heads#L603-605 #L604 for details regarding the /metrics endpoint.

For configuration instructions, see: https://github.com/backstage/backstage/blob/master/contrib/docs/tutorials/prometheus-metrics.md