Small file read performance improvement for DirectFileStore

Instead of two `read` operations, we can do both together at once

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -257,8 +257,7 @@ def all_values
             with_file_lock do
               @positions.map do |key, pos|
                 @f.seek(pos)
-                value = @f.read(8).unpack('d')[0]
-                timestamp = @f.read(8).unpack('d')[0]
+                value, timestamp = @f.read(16).unpack('dd')
                 [key, value, timestamp]
               end
             end