pkg/ruler/rulestore/configdb: Preallocate memory (#7326)

Preallocate memory instead of enforcing an incremental growth. This will
result in less work for the garbage collector.

pkg/ruler/rulestore/configdb/store.go

@@ -36,8 +36,7 @@ func NewConfigRuleStore(c client.Client) *ConfigRuleStore {
 func (c *ConfigRuleStore) ListAllUsers(ctx context.Context) ([]string, error) {
 	m, err := c.ListAllRuleGroups(ctx)
 
-	// TODO: this should be optimized, if possible.
-	result := []string(nil)
+	result := make([]string, 0, len(m))
 	for u := range m {
 		result = append(result, u)
 	}