Fix WithGroup metric option being silently ignored

[riptide-01]

Description

operation.WithGroup() is the only public way for a Go hook to assign a metric to a group, but it never actually took effect. The option method was declared on a value receiver, so the assignment mutated a copy of the operation and was thrown away, even though the collector correctly applies options through a pointer.

Tests

func handlerHook(_ context.Context, input *pkg.HookInput) error {
	input.MetricsCollector.Set(metricName, 1, map[string]string{"test": "1"}, operation.WithGroup(metricGroup))
	input.MetricsCollector.Set(metricName2, 2, map[string]string{"test": "2"})
	input.MetricsCollector.Expire(metricGroup)
	return nil
}

Before:

After: