✨ metrics: add generator for kube-state-metrics customresource configuration

[chrischdi]

This adds a generator to generate a customresource configuration file for kube-state-metrics from markers.

With that configuration it is possible to get metrics for CRs like kube-state-metrics exposes for core resources.
(xref)

Example usage:

controller-gen metrics output:metrics:stdout \
     paths=./api/...\
     > crd-config.yaml

Introduced markers:

$ controller-gen metrics -ww

Metric type Gauge


+Metrics:gauge type
	defines a Gauge metric and uses the implicit path to the field joined by the provided JSONPath as path for the metric configuration.
	[JSONPath=<string>]      specifies the relative path from this marker.                                          
	[help=<string>]          specifies the help text for the metric.                                                
	[labelFromKey=<string>]  specifies a label which will be added to the metric having the object's key as value.  
	[labelsFromPath=<>]      specifies additional labels where the value is taken from the given JSONPath.          
	name=<string>            specifies the Name of the metric.                                                      
	[nilIsZero=<bool>]       specifies to treat a not-existing field as zero value.                                 
	[valueFrom=<string>]     specifies the JSONPath to a numeric field that will be the metric value.               

+Metrics:gauge field
	defines a Gauge metric and uses the implicit path to the field joined by the provided JSONPath as path for the metric configuration.
	[JSONPath=<string>]      specifies the relative path from this marker.                                          
	[help=<string>]          specifies the help text for the metric.                                                
	[labelFromKey=<string>]  specifies a label which will be added to the metric having the object's key as value.  
	[labelsFromPath=<>]      specifies additional labels where the value is taken from the given JSONPath.          
	name=<string>            specifies the Name of the metric.                                                      
	[nilIsZero=<bool>]       specifies to treat a not-existing field as zero value.                                 
	[valueFrom=<string>]     specifies the JSONPath to a numeric field that will be the metric value.               

Metric type Info


+Metrics:info type
	defines a Info metric and uses the implicit path to the field as path for the metric configuration.
	[JSONPath=<string>]      specifies the relative path from this marker.                                          
	[help=<string>]          specifies the help text for the metric.                                                
	[labelFromKey=<string>]  specifies a label which will be added to the metric having the object's key as value.  
	[labelsFromPath=<>]      specifies additional labels where the value is taken from the given JSONPath.          
	name=<string>            specifies the Name of the metric.                                                      

+Metrics:info field
	defines a Info metric and uses the implicit path to the field as path for the metric configuration.
	[JSONPath=<string>]      specifies the relative path from this marker.                                          
	[help=<string>]          specifies the help text for the metric.                                                
	[labelFromKey=<string>]  specifies a label which will be added to the metric having the object's key as value.  
	[labelsFromPath=<>]      specifies additional labels where the value is taken from the given JSONPath.          
	name=<string>            specifies the Name of the metric.                                                      

Metric type StateSet


+Metrics:stateset type
	defines a StateSet metric and uses the implicit path to the field as path for the metric configuration.
	[JSONPath=<string>]   specifies the path to the field which gets used as value to compare against the list for equality.  
	[help=<string>]       specifies the help text for the metric.                                                             
	[labelName=<string>]  specifies the key of the label which is used for each entry in List to expose the value.            
	[labelsFromPath=<>]   specifies additional labels where the value is taken from the given JSONPath.                       
	list=<[]string>       specifies a list of values to compare the given JSONPath against.                                   
	name=<string>         specifies the Name of the metric.                                                                   

+Metrics:stateset field
	defines a StateSet metric and uses the implicit path to the field as path for the metric configuration.
	[JSONPath=<string>]   specifies the path to the field which gets used as value to compare against the list for equality.  
	[help=<string>]       specifies the help text for the metric.                                                             
	[labelName=<string>]  specifies the key of the label which is used for each entry in List to expose the value.            
	[labelsFromPath=<>]   specifies additional labels where the value is taken from the given JSONPath.                       
	list=<[]string>       specifies a list of values to compare the given JSONPath against.                                   
	name=<string>         specifies the Name of the metric.                                                                   

Metrics


+Metrics:gvk type
	enables the creation of a custom resource configuration entry and uses the given prefix for the metrics if configured.
	[namePrefix=<string>]  specifies the prefix for all metrics of this resource.  

+Metrics:labelFromPath type
	specifies additional labels for all metrics of this field or type.
	JSONPath=<string>  specifies the relative path to the value for the label.  
	name=<string>      specifies the name of the label.                         

+Metrics:labelFromPath field
	specifies additional labels for all metrics of this field or type.
	JSONPath=<string>  specifies the relative path to the value for the label.  
	name=<string>      specifies the name of the label.                         

Example usage: kubernetes-sigs/cluster-api#9347

[chrischdi]

If wanted, I can volunteer to maintain this part of controller-gen (and/or also help on other parts of controller-tools) :-)

[sbueringer]

We'll get back to this eventually, just low bandwith at the moment

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[sbueringer]

/remove-lifecycle stale

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[chrischdi]

/reopen

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[chrischdi]

/remove-lifecycle rotten

[sbueringer]

@JoelSpeed If you're fine with this in general (already discussed this with Vince+Alvaro in the past and they are okay with it as long as we mark it experimental in some way for now) + we all have time to push this towards completion. Happy to invest time in reviews

[JoelSpeed]

I'm not going to have time to do a deep dive on this until next week, but at the high level this seems like a reasonable addition.

Would like to have some discussion (maybe at the DV subproject meeting) about precedence for similar markers, and the patterns for defining the markers so that we can be somewhat consistent

[sbueringer]

I'm not going to have time to do a deep dive on this until next week, but at the high level this seems like a reasonable addition.

Sounds good, no rush!

Would like to have some discussion (maybe at the DV subproject meeting) about precedence for similar markers, and the patterns for defining the markers so that we can be somewhat consistent

Sounds reasonable. @chrischdi would be good if you could bring this up and drive forward.
Depending on bandwidth of course (don't want to just assume that you currently have the right bandwidth/priority mix for this :)). The meeting Joel is referring to is this one: https://docs.google.com/document/d/1cYXfDHZpalzoew-PKfgh1XHQB_6JjiEsidsM4zYGAzc/edit?pli=1&tab=t.0#heading=h.xf24wnv1ochq (bi-weekly Tuesday, next ocurrence next week)