Cherry-pick #23905 to 7.x: [Metricbeat][Kubernetes] Extend state_node with more conditions

[ChrsMark]

Cherry-pick of PR #23905 to 7.x branch. Original message:

What does this PR do?

This PR extends the state_node module of the Metricbeat's Kubernetes module with more status conditions, such as MemoryPressure, DiskPressure, OutOfDisk and PIDPressure.

Refactor the opFilter Prometheus metric helper to allow the declaration of multiple Metricbeat fields from a single Prometheus metric, based on allowed label values.

Why is it important?

Adding more status conditions of Kubernetes nodes is useful for monitoring and alerting purposes as it allows one to use conditions like:

• Is any node running out of memory?
• Is any node running out of disk?
• Is any node running out of PIDs?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [Metricbeat] Add support for node conditions to state_node metricset #18049

Use cases

Screenshots

Logs

2021-02-08T12:34:38.470+0200    DEBUG   [processors]    processing/processors.go:203    Publish event: {                                                                                                                                      
  "@timestamp": "2021-02-08T10:34:38.307Z",                                                                                                                                                                                                   
  "@metadata": {                                                                                                                                                                                                                              
    "beat": "metricbeat",                                                                                                                                                                                                                     
    "type": "_doc",                                                                                                                                                                                                                           
    "version": "8.0.0"                                                                                                                                                                                                                        
  },                                                                                                                                                                                                                                          
  "event": {                                                                                                                                                                                                                                  
    "module": "kubernetes",                                                                                                                                                                                                                   
    "duration": 162891130,                                                                                                                                                                                                                    
    "dataset": "kubernetes.node"                                                                                                                                                                                                              
  },                                                                                                                                                                                                                                          
  "kubernetes": {                                                                                                                                                                                                                             
    "node": {                                                                                                                                                                                                                                 
      "name": "node1",                                                                                                                                                                                                                      
      "status": {                                                                                                                                                                                                                             
        "unschedulable": false,                                                                                                                                                                                                               
        "memorypressure": "false",                                                                                                                                                                                                            
        "diskpressure": "false",                                                                                                                                                                                                              
        "pidpressure": "false",                                                                                                                                                                                                               
        "ready": "true"                                                                                                                                                                                                                       
      }                                                                                                                                                                                                                                       
    }                                                                                                                                                                                                                                         
  },                                                                                                                                                                                                                                          
  "ecs": {                                                                                                                                                                                                                                    
    "version": "1.7.0"                                                                                                                                                                                                                        
  },  
...

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #24057 opened

• Start Time: 2021-02-16T12:34:59.140+0000

• Duration: 34 min 8 sec

• Commit: 85359a3

Test stats 🧪

Test	Results
Failed	0
Passed	8658
Skipped	2122
Total	10780

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	8658
Skipped	2122
Total	10780