perf: improve query performance for Metric.latest

Currently it pulls all records from the database but only uses the most recent amount value.

This is inefficient, particularly if the database is large. Now it only retrieves the amount from
the most recent record and ignores all the old ones.

lib/metricks/models/metric.rb

@@ -67,7 +67,7 @@ def record(type, **options)
         # @return [Float]
         def latest(type, **options)
           scope = self.last(type, **options)
-          value = scope.pluck(:amount)&.first || 0.0
+          value = scope.select(:amount).first&.amount || 0.0
           type.transform_amount(value, options[:associations])
         end