[cmd/mdatagen]: Add feature gates support to metadata-schema.yaml

[narcis96]

Description:
Added feature-gates section to metadata-schema.yaml together with id, description, stage and other required. This information can then be used for code generation, documentation generation and by other consumers of the metadata.yaml file

Link to tracking Issue: #9860

[mx-psi]

Thanks! Left some comments

[mx-psi]

We are missing the code generation! We need to generate something like this:

opentelemetry-collector/featuregate/README.md

Lines 19 to 25 in 388bae7

	var myFeatureGate = featuregate.GlobalRegistry().MustRegister(
	"namespaced.uniqueIdentifier",
	featuregate.Stable,
	featuregate.WithRegisterFromVersion("v0.65.0")
	featuregate.WithRegisterDescription("A brief description of what the gate controls"),
	featuregate.WithRegisterReferenceURL("https://github.com/open-telemetry/opentelemetry-collector/issues/6167"),
	featuregate.WithRegisterToVersion("v0.70.0"))

[mx-psi]

We should also check for stage, that is also a required field

[mx-psi]

All of these should check first where the field is set, otherwise they are effectively required, aren't they?

For example, instead of:

if !versionRegexp.MatchString(fmt.Sprintf("%v", parser.Get("from_version"))) {
	err = append(err, fmt.Errorf("invalid character(s) in from_version"))
}

we should do something similar to this

if parser.IsSet("from_version") && !versionRegexp.MatchString(fmt.Sprintf("%v", parser.Get("from_version"))) {
	err = append(err, fmt.Errorf("invalid character(s) in from_version"))
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.

Project coverage is 92.03%. Comparing base (a7d019f) to head (05ec7bf).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/loader.go	0.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11466      +/-   ##
==========================================
- Coverage   92.15%   92.03%   -0.12%     
==========================================
  Files         432      433       +1     
  Lines       20253    20412     +159     
==========================================
+ Hits        18664    18787     +123     
- Misses       1228     1261      +33     
- Partials      361      364       +3     

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