Allow to specify a custom scope in Rackstash::Fields::Array#concat

lib/rackstash/fields/array.rb

@@ -67,9 +67,11 @@ module Rackstash
       #
       # @param array [Array, ::Array] an array of values. Each value is
       #   normalized before being added to `self`.
+      # @param scope [Object] if `array` or any of its (deeply-nested) values is
+      #   a proc, it will be called in the instance scope of this object.
       # @return [self]
-      def concat(array)
-        array = Array(normalize(array, wrap: false))
+      def concat(array, scope: nil)
+        array = Array(normalize(array, wrap: false, scope: scope))
         @raw.concat(array)
         self
       end

spec/rackstash/fields/array_spec.rb

@@ -143,6 +143,12 @@ describe Rackstash::Fields::Array do
     it 'resolves nested procs' do
       expect(array.concat(-> { [-> { :foo } ] } )).to contain_exactly 'foo'
     end
+
+    it 'resolves nested procs with a custom scope' do
+      expect(
+        array.concat(-> { [self, -> { self.to_s.upcase } ] }, scope: :stuff )
+      ).to contain_exactly 'stuff', 'STUFF'
+    end
   end
 
   describe '#empty?' do