v1.18.2-fix-union-selection

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

v1.18.1-fix-union-selection

bug fix for subscription usage info unmarshalling when group by is used