Add status endpoint to seperate webserver

The /internal/v4/status endpoint gives more fine-granular insights in the health state of the CC.
When at leats one Puma worker is idling it returns OK. If all workers are occupied, it returns BUSY.
If all workers are busy and since 60s no requests have been processed, it returns UNHEALTHY.
This endpoint will be used to prevent restarts even if CC is still healthy and working off peak-loads.

Author: svkrieger

lib/cloud_controller/runner.rb

@@ -136,12 +136,17 @@ def setup_metrics
       setup_metrics_webserver
     end
 
-    # The webserver runs in the main process and serves only the metrics endpoint.
-    # This makes it possible to retrieve metrics even if all Puma workers of the main app are busy.
+    # The webserver runs in the main process and serves only the metrics and status endpoint.
+    # This makes it possible to retrieve both even if all Puma workers of the main app are busy.
     def setup_metrics_webserver
+      readiness_status_proc = method(:status)
       metrics_app = Rack::Builder.new do
         use Prometheus::Middleware::Exporter, path: '/internal/v4/metrics'
 
+        map '/internal/v4/status' do
+          run ->(_env) { readiness_status_proc.call }
+        end
+
         map '/' do
           run lambda { |_env|
             # Return 404 for any other request
@@ -163,6 +168,52 @@ def setup_metrics_webserver
       end
     end
 
+    # Persist state for status endpoint
+    @previous_requests_count_sum = nil
+    @last_requests_count_increase_time = nil
+
+    def status
+      stats = Puma.stats_hash
+      worker_statuses = stats[:worker_status]
+      all_busy = all_workers_busy?(worker_statuses)
+      current_requests_count_sum = worker_requests_count_sum(worker_statuses)
+
+      now = Time.now
+      prev = @previous_requests_count_sum
+
+      # Track when requests_count_sum increases
+      @last_requests_count_increase_time = now if prev.nil? || current_requests_count_sum > prev
+      @previous_requests_count_sum = current_requests_count_sum
+
+      unhealthy = false
+      if all_busy && @last_requests_count_increase_time && (now - @last_requests_count_increase_time) > 60
+        # If requests_count_sum hasn't increased in 60 seconds, unhealthy
+        unhealthy = true
+      end
+
+      if all_busy && unhealthy
+        [503, { 'Content-Type' => 'text/plain' }, ['UNHEALTHY']]
+      elsif all_busy
+        [429, { 'Content-Type' => 'text/plain' }, ['BUSY']]
+      else
+        [200, { 'Content-Type' => 'text/plain' }, ['OK']]
+      end
+    rescue StandardError => e
+      [500, { 'Content-Type' => 'text/plain' }, ["Readiness check error: #{e}"]]
+    end
+
+    def all_workers_busy?(worker_statuses)
+      worker_statuses.all? do |worker|
+        worker[:last_status][:busy_threads] == worker[:last_status][:running]
+      end
+    end
+
+    def worker_requests_count_sum(worker_statuses)
+      worker_statuses.sum do |worker|
+        worker[:last_status][:requests_count] || 0
+      end
+    end
+
     def setup_logging
       return if @setup_logging