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