Support for application-specific metrics tracking

[devdattakulkarni]

Currently, KubePlus tracks the following metrics by default: CPU, memory, ingress network, egress network, and storage. We should support the ability to track application-specific metrics through ResourceMonitor CRD. The assumption here will be that the application exposes such metrics at some endpoint that KubePlus is able to scrape and surface out for ingesting into Prometheus.

[devdattakulkarni]

The changes would be to following:

• Adding a field to ResourceMonitor to define the application endpoint.
• Updating the kubectl-metrics plugin to query this endpoint for the input Kind/Instance, and surface it in the output.

To test this functionality we will need to create an application that collects some metrics and exposes them on such an endpoint.