Add req count & req latency metrics for UI

ui/config.ru

@@ -2,7 +2,9 @@ require './ui_app'
 require 'rack'
 require 'prometheus/middleware/collector'
 require 'prometheus/middleware/exporter'
+require './middleware.rb'
 
+use Metrics
 use Rack::Deflater, if: ->(_, _, _, body) { body.any? && body[0].length > 512 }
 use Prometheus::Middleware::Collector
 use Prometheus::Middleware::Exporter

ui/middleware.rb

@@ -0,0 +1,23 @@
+require 'prometheus/client'
+
+class Metrics
+
+  def initialize app
+    @app = app
+    prometheus = Prometheus::Client.registry
+    @request_count = Prometheus::Client::Counter.new(:ui_request_count, 'App Request Count')
+    @request_latency = Prometheus::Client::Histogram.new(:ui_request_latency_seconds, 'Request latency')
+    prometheus.register(@request_latency)
+    prometheus.register(@request_count)
+  end
+
+  def call env
+    request_started_on = Time.now
+    @status, @headers, @response = @app.call(env)
+    request_ended_on = Time.now
+    @request_latency.observe({ path: env['REQUEST_PATH'] }, request_ended_on - request_started_on)
+    @request_count.increment({ method: env['REQUEST_METHOD'], path: env['REQUEST_PATH'], http_status: @status })
+    [@status, @headers, @response]
+  end
+
+end