Fix subscription usage union selection bug for grouped responses

Add CustomMatcher support to apijson union system to properly handle
subscription usage responses where grouped and ungrouped variants both
match gjson.JSON TypeFilter.

The issue was that both SubscriptionUsageGroupedSubscriptionUsage and
SubscriptionUsageUngroupedSubscriptionUsage used identical TypeFilter
values, causing incorrect variant selection based on ordering rather
than content.

Solution:
- Extended UnionVariant with CustomMatcher function field
- Modified union decoder to check CustomMatcher before TypeFilter
- Used CustomMatcher to detect presence of metric_group field in data
- Ensures grouped responses select grouped variant, ungrouped select ungrouped

Fixes: Analytics functionality that depends on GroupBy parameter
Affects: All grouped usage requests in subscription usage API
Versions: Confirmed broken in v1.4.1 through v1.18.0

Author: Cameron Custer

internal/apijson/decoder.go

@@ -219,6 +219,18 @@ func (d *decoderBuilder) newUnionDecoder(t reflect.Type) decoderFunc {
 		// If there is a discriminator match, circumvent the exactness logic entirely
 		for idx, variant := range unionEntry.variants {
 			decoder := decoders[idx]
+
+			// Check custom matcher first
+			if variant.CustomMatcher != nil {
+				if variant.CustomMatcher([]byte(n.Raw)) {
+					inner := reflect.New(variant.Type).Elem()
+					err := decoder(n, inner, state)
+					v.Set(inner)
+					return err
+				}
+				continue
+			}
+
 			if variant.TypeFilter != n.Type {
 				continue
 			}

internal/apijson/registry.go

@@ -10,6 +10,9 @@ type UnionVariant struct {
 	TypeFilter         gjson.Type
 	DiscriminatorValue interface{}
 	Type               reflect.Type
+	// CustomMatcher is an optional function that can be used for custom matching logic
+	// If provided, it takes precedence over TypeFilter and DiscriminatorValue
+	CustomMatcher func([]byte) bool
 }
 
 var unionRegistry = map[reflect.Type]unionEntry{}

subscription.go

@@ -5277,12 +5277,15 @@ func init() {
 		reflect.TypeOf((*SubscriptionUsageUnion)(nil)).Elem(),
 		"",
 		apijson.UnionVariant{
-			TypeFilter: gjson.JSON,
-			Type:       reflect.TypeOf(SubscriptionUsageUngroupedSubscriptionUsage{}),
+			Type: reflect.TypeOf(SubscriptionUsageGroupedSubscriptionUsage{}),
+			CustomMatcher: func(data []byte) bool {
+				// Use grouped variant if metric_group field exists in any data element
+				return gjson.GetBytes(data, "data.0.metric_group").Exists()
+			},
 		},
 		apijson.UnionVariant{
 			TypeFilter: gjson.JSON,
-			Type:       reflect.TypeOf(SubscriptionUsageGroupedSubscriptionUsage{}),
+			Type:       reflect.TypeOf(SubscriptionUsageUngroupedSubscriptionUsage{}),
 		},
 	)
 }