Reduce memory bloating in FileMappedDict when reading metrics.

During the read path (metrics export) we open all existing metric files
and read through them in order to aggregate metrics across different
processes. When reading non-empty files we end up parsing file content
twice: first during FileMappedDict initialisation, then again in the
caller site (`all_values`).

This commit refactors FileMappedDict so that while the `@positions` map
is populated at creation time, the actual metric values are read only
when explicitly requested. This avoids the memory bloat of unpacking
file content twice.

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -215,11 +215,7 @@ def initialize(filename, readonly = false)
 
             if @used > 0
               # File already has data. Read the existing values
-              with_file_lock do
-                read_all_values.each do |key, _, pos|
-                  @positions[key] = pos
-                end
-              end
+              with_file_lock { populate_positions }
             else
               # File is empty. Init the `used` counter, if we're in write mode
               if !readonly
@@ -230,10 +226,14 @@ def initialize(filename, readonly = false)
             end
           end
 
-          # Yield (key, value, pos). No locking is performed.
+          # Return a list of key-value pairs
           def all_values
             with_file_lock do
-              read_all_values.map { |k, v, p| [k, v] }
+              @positions.map do |key, pos|
+                @f.seek(pos)
+                value = @f.read(8).unpack('d')[0]
+                [key, value]
+              end
             end
           end
 
@@ -309,22 +309,18 @@ def init_value(key)
             @positions[key] = @used - 8
           end
 
-          # Yield (key, value, pos). No locking is performed.
-          def read_all_values
+          # Read position of all keys. No locking is performed.
+          def populate_positions
             @f.seek(8)
-            values = []
             while @f.pos < @used
               padded_len = @f.read(4).unpack('l')[0]
-              encoded = @f.read(padded_len).unpack("A#{padded_len}")[0]
-              value = @f.read(8).unpack('d')[0]
-              values << [encoded.strip, value, @f.pos - 8]
+              key = @f.read(padded_len).unpack("A#{padded_len}")[0].strip
+              @positions[key] = @f.pos
+              @f.seek(8, :CUR)
             end
-            values
           end
         end
       end
     end
   end
 end
-
-