chore: add rack middleware for tracking response status codes

README.md

@@ -10,6 +10,7 @@ As well as providing a semantic logger, this gem handles datadog telemetry assoc
 * traces
 * metrics
     * statsd is automatically attached to datadog runtime metrics and may also be used for custom metrics.
+    * `ResponseCodeStatsMiddleware` is included to capture response code metrics from rack applications
 
 ### Customise log level per logger
 
@@ -60,6 +61,13 @@ For example, to increment a count:
 NexusSemanticLogger.metrics.increment('nexus.users.registration.complete')
 ```
 
+### Rack response code metrics
+This can be configured with a middleware in application.rb
+
+```
+config.middleware.use ResponseCodeStatsMiddleware
+```
+
 # Local gem development
 
 Steps to run this gem from local sources in one the nexus 'staged build' rails components:

lib/puma/plugin/nexus_puma_statsd.rb

@@ -164,6 +164,10 @@ Puma::Plugin.create do
         NexusSemanticLogger.metrics.gauge('puma.pool_capacity', stats.pool_capacity, tags: tags)
         NexusSemanticLogger.metrics.gauge('puma.max_threads', stats.max_threads, tags: tags)
         NexusSemanticLogger.metrics.gauge('puma.requests_count', stats.requests_count, tags: tags)
+
+        ResponseCodeStatsMiddleware.read_and_reset_metrics.each do |code, count|
+          NexusSemanticLogger.metrics.gauge("puma.response_code.#{code}", count, tags: tags)
+        end
       rescue StandardError => e
         @log_writer.unknown_error(e, nil, '! statsd: notify stats failed')
       ensure

lib/rack/response_code_stats_middleware.rb

+# frozen_string_literal: true
+
+# noinspection RubyClassVariableUsageInspection
+class ResponseCodeStatsMiddleware
+  def initialize(app)
+    @app = app
+    @@code_metrics = {}
+  end
+
+  def call(env)
+    status, headers, response = @app.call(env)
+
+    @@code_metrics[status] ||= 0
+    @@code_metrics[status] += 1
+
+    [status, headers, response]
+  end
+
+  def self.read_and_reset_metrics
+    metrics = @@code_metrics.dup
+    @@code_metrics.clear
+    metrics
+  end
+end