Add Passthrough transform and formatter

These are useful for custom targets that might not need/want any
transforms for formatting, or to mix-and-match. For example, image a
custom `LogTarget` that can be configured with a `:logger` option. It
could be that the logger itself already knows how to format they
telemetry data, (e.g., into key/value pairs), and so we don't want/need
the Target to do any formatting.

This allows this plugin's config to look like this:

```ruby
Puma::Plugin::Telemetry.configure do |config|
  config.add_target(CustomLogTarget, logger: Logfmt.logger, transform: :cloud_watch, formatter: :noop)
end
```

lib/puma/plugin/telemetry/formatters/json_formatter.rb

@@ -6,7 +6,7 @@ module Puma
   class Plugin
     module Telemetry
       module Formatters
-        # JSON formatter for IO, expects `call` method accepting telemetry hash
+        # JSON formatter, expects `call` method accepting telemetry hash
         #
         class JSONFormatter
           def self.call(telemetry)

lib/puma/plugin/telemetry/formatters/passthrough_formatter.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Puma
+  class Plugin
+    module Telemetry
+      module Formatters
+        # A passthrough formatter - it returns the telemetry Hash it was given
+        class PassthroughFormatter
+          def self.call(telemetry)
+            telemetry
+          end
+        end
+      end
+    end
+  end
+end

lib/puma/plugin/telemetry/targets/base_formatting_target.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require_relative '../formatters/json_formatter'
+require_relative '../formatters/passthrough_formatter'
+require_relative '../transforms/cloud_watch_transform'
+require_relative '../transforms/passthrough_transform'
+
+module Puma
+  class Plugin
+    module Telemetry
+      module Targets
+        # A base class for other Targets concerned with formatting telemetry
+        class BaseFormattingTarget
+          def initialize(formatter: :json, transform: :cloud_watch)
+            @transform = case transform
+                         when :cloud_watch then Transforms::CloudWatchTranform
+                         when :passthrough then Transforms::PassthroughTransform
+                         else transform
+                         end
+            @formatter = case formatter
+                         when :json then Formatters::JSONFormatter
+                         when :passthrough then Formatters::PassthroughFormatter
+                         else formatter
+                         end
+          end
+
+          def call(_telemetry)
+            raise "#{__method__} must be implemented by #{self.class.name}"
+          end
+
+          private
+
+          attr_reader :formatter, :transform
+        end
+      end
+    end
+  end
+end

lib/puma/plugin/telemetry/targets/io_target.rb

@@ -1,25 +1,16 @@
 # frozen_string_literal: true
 
-require_relative '../formatters/json_formatter'
-require_relative '../transforms/cloud_watch_transform'
+require_relative 'base_formatting_target'
 
 module Puma
   class Plugin
     module Telemetry
       module Targets
         # Simple IO Target, publishing metrics to STDOUT or logs
-        #
-        class IOTarget
+        class IOTarget < BaseFormattingTarget
           def initialize(io: $stdout, formatter: :json, transform: :cloud_watch)
+            super(formatter: formatter, transform: transform)
             @io = io
-            @transform = case transform
-                         when :cloud_watch then Transforms::CloudWatchTranform
-                         else transform
-                         end
-            @formatter = case formatter
-                         when :json then Formatters::JSONFormatter
-                         else formatter
-                         end
           end
 
           def call(telemetry)
@@ -28,7 +19,7 @@ def call(telemetry)
 
           private
 
-          attr_reader :formatter, :io, :transform
+          attr_reader :io
         end
       end
     end

lib/puma/plugin/telemetry/transforms/passthrough_transform.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Puma
+  class Plugin
+    module Telemetry
+      module Transforms
+        # A passthrough transform - it returns the telemetry Hash it was given
+        class PassthroughTransform
+          def self.call(telemetry)
+            telemetry
+          end
+        end
+      end
+    end
+  end
+end

spec/fixtures/sockets.rb

@@ -21,7 +21,7 @@
 
 Puma::Plugin::Telemetry.configure do |config|
   # Simple `key=value` formatter
-  config.add_target(:io, formatter: ->(t) { t.map { |r| r.join('=') }.join(' ') }, transform: ->(t) { t })
+  config.add_target(:io, formatter: ->(t) { t.map { |r| r.join('=') }.join(' ') }, transform: :passthrough)
   config.frequency = 1
   config.enabled = true
 

spec/puma/plugin/telemetry/formatters/passthrough_formatter_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Puma
+  class Plugin
+    module Telemetry
+      module Formatters
+        RSpec.describe PassthroughFormatter do
+          subject(:formatter) { described_class }
+
+          it 'returns the telemetry, unalterted' do
+            telmetry_data = { 'foo' => 'bar' }
+            formatted_data = formatter.call(telmetry_data)
+
+            expect(formatted_data).to eq(telmetry_data)
+          end
+        end
+      end
+    end
+  end
+end

spec/puma/plugin/telemetry/transforms/passthrough_transform_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Puma
+  class Plugin
+    module Telemetry
+      module Transforms
+        RSpec.describe PassthroughTransform do
+          subject(:transform) { described_class }
+
+          it 'returns the telemetry, unalterted' do
+            telmetry_data = { 'foo' => 'bar' }
+            transformed_data = transform.call(telmetry_data)
+
+            expect(transformed_data).to eq(telmetry_data)
+          end
+        end
+      end
+    end
+  end
+end