chore: add descheduler plugin example

[ricardomaraschini]

This commit adds a sample plugin implementation as follow:

Descheduler Plugin: Example Implementation

This directory provides an example plugin for the Kubernetes Descheduler, demonstrating how to evict pods based on custom criteria. The plugin targets pods based on:

• Name Regex: Pods matching a specified regular expression.
• Age: Pods older than a defined duration.
• Namespace: Pods within or outside a given list of namespaces (inclusion or exclusion).

Building and Integrating the Plugin

To incorporate this plugin into your Descheduler build, you must register it within the Descheduler's plugin registry. Follow these steps:

1. Register the Plugin:

   • Modify the pkg/descheduler/setupplugins.go file.
   • Add the following registration line to the end of the RegisterDefaultPlugins() function:

   pluginregistry.Register(
     example.PluginName,
     example.New,
     &example.Example{},
     &example.ExampleArgs{},
     example.ValidateExampleArgs,
     example.SetDefaults_Example,
     registry,
   )

2. Generate Code:

   • If you modify the plugin's code, execute make gen before rebuilding the Descheduler. This ensures generated code is up-to-date.

3. Rebuild the Descheduler:

   • Build the descheduler with your changes.

Plugin Configuration

Configure the plugin's behavior using the Descheduler's policy configuration. Here's an example:

apiVersion: descheduler/v1alpha2
kind: DeschedulerPolicy
profiles:
- name: LifecycleAndUtilization
  plugins:
    deschedule:
      enabled:
        - Example
  pluginConfig:
  - name: Example
    args:
      regex: ^descheduler-test.*$
      maxAge: 3m
      namespaces:
        include:
        - default

Explanation

• regex: ^descheduler-test.*$: Evicts pods whose names match the regular expression ^descheduler-test.*$.
• maxAge: 3m: Evicts pods older than 3 minutes.
• namespaces.include: - default: Evicts pods within the default namespace.

This configuration will cause the plugin to evict pods that meet all three criteria: matching the regex, exceeding the maxAge, and residing in the specified namespace.

Notes

• This plugin is configured through the ExampleArgs struct, which defines the plugin's parameters.
• Plugins must implement a function to validate and another to set the default values for their Args struct.
• The fields in the ExampleArgs struct reflect directly into the DeschedulerPolicy configuration.
• Plugins must comply with the DeschedulePlugin interface to be registered with the Descheduler.
• The main functionality of the plugin is implemented in the Deschedule() method, which is called by the Descheduler when the plugin is executed.
• A good amount of descheduling logic can be achieved by means of filters.
• Whenever a change in the Plugin's configuration is made the developer should regenerate the code by running make gen.

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[ricardomaraschini]

This should address #923

[googs1025]

/cc

[ingvagabund]

/ok-to-test

[ingvagabund]

This looks really great :) Thank you.
/approve
/lgtm

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