Add PR reference to TODO comment about LabelNames replacement (#15)

* 💡(metadata_query): Add PR reference to TODO comment about LabelNames replacement
[Explanation: Added GitHub PR link (#1850) to existing TODO comment to provide context for future type replacement from []string to model.LabelNames]

* 💡(labels.go): Improve TODO comment clarity and formatting

```diff
-// TODO: replace []string with model.LabelNames ? prometheus/client_golang#1850
+// TODO: Replace []string with model.LabelNames (see prometheus/client_golang#1850).
```

Author: yshngg

pkg/metadata_query/labels.go

@@ -28,7 +28,7 @@ type LabelNamesArguments struct {
 }
 
 type LabelNamesResult struct {
-	// TODO: replace []string with model.LabelNames ?
+	// TODO: Replace []string with model.LabelNames (see https://github.com/prometheus/client_golang/pull/1850).
 	LabelNames []string    `json:"names" jsonschema:"Names is a list of string label names."`
 	Warnings   v1.Warnings `json:"warnings,omitempty"`
 }