FEATURE: Introduce http_application_duration_seconds summary metric (…

…#67)

The metric is useful when introducing a change that would only affect
the performance of application code. For example, upgrading Ruby
versions or enabling YJIT in Ruby. With this metric, we are able to
isolate and analyse the performance of the application code as a result
of those changes.

lib/collector.rb

@@ -13,6 +13,7 @@ def initialize
       @page_views = nil
       @http_requests = nil
       @http_duration_seconds = nil
+      @http_application_duration_seconds = nil
       @http_redis_duration_seconds = nil
       @http_sql_duration_seconds = nil
       @http_net_duration_seconds = nil
@@ -226,12 +227,25 @@ def ensure_web_metrics
 
         @http_duration_seconds =
           Summary.new("http_duration_seconds", "Time spent in HTTP reqs in seconds")
+
+        @http_application_duration_seconds =
+          Summary.new(
+            "http_application_duration_seconds",
+            "Time spent in application code within HTTP reqs in seconds",
+          )
+
         @http_redis_duration_seconds =
-          Summary.new("http_redis_duration_seconds", "Time spent in HTTP reqs in redis seconds")
+          Summary.new(
+            "http_redis_duration_seconds",
+            "Time spent in Redis within HTTP reqs redis seconds",
+          )
+
         @http_sql_duration_seconds =
-          Summary.new("http_sql_duration_seconds", "Time spent in HTTP reqs in SQL in seconds")
+          Summary.new("http_sql_duration_seconds", "Time spent in SQL within HTTP reqs in seconds")
+
         @http_net_duration_seconds =
           Summary.new("http_net_duration_seconds", "Time spent in external network requests")
+
         @http_queue_duration_seconds =
           Summary.new(
             "http_queue_duration_seconds",
@@ -274,7 +288,18 @@ def process_web(metric)
         labels[:action] = "other"
       end
 
-      @http_duration_seconds.observe(metric.duration, labels)
+      duration = metric.duration
+
+      @http_duration_seconds.observe(duration, labels)
+
+      if duration
+        application_duration = duration.dup
+        application_duration -= metric.sql_duration if metric.sql_duration
+        application_duration -= metric.redis_duration if metric.redis_duration
+        application_duration -= metric.net_duration if metric.net_duration
+        @http_application_duration_seconds.observe(application_duration, labels)
+      end
+
       @http_sql_duration_seconds.observe(metric.sql_duration, labels)
       @http_redis_duration_seconds.observe(metric.redis_duration, labels)
       @http_net_duration_seconds.observe(metric.net_duration, labels)
@@ -388,6 +413,7 @@ def web_metrics
           @page_views,
           @http_requests,
           @http_duration_seconds,
+          @http_application_duration_seconds,
           @http_redis_duration_seconds,
           @http_sql_duration_seconds,
           @http_net_duration_seconds,

spec/lib/collector_spec.rb

@@ -253,10 +253,10 @@ module DiscoursePrometheus
 
       metrics << InternalMetric::Web.get(
         status_code: 200,
-        duration: 5,
+        duration: 10,
         sql_duration: 1,
-        redis_duration: 3,
-        net_duration: 5,
+        redis_duration: 2,
+        net_duration: 3,
         json: true,
         controller: "list",
         action: "latest",
@@ -265,10 +265,10 @@ module DiscoursePrometheus
 
       metrics << InternalMetric::Web.get(
         status_code: 302,
-        duration: 5,
+        duration: 10,
         sql_duration: 1,
-        redis_duration: 3,
-        net_duration: 5,
+        redis_duration: 2,
+        net_duration: 3,
         controller: "list",
         action: "latest",
         cache: true,
@@ -311,10 +311,11 @@ module DiscoursePrometheus
       }
 
       [
-        ["http_duration_seconds", 5.0],
+        ["http_duration_seconds", 10.0],
+        ["http_application_duration_seconds", 4.0],
         ["http_sql_duration_seconds", 1.0],
-        ["http_redis_duration_seconds", 3.0],
-        ["http_net_duration_seconds", 5.0],
+        ["http_redis_duration_seconds", 2.0],
+        ["http_net_duration_seconds", 3.0],
       ].each { |metric_name, sum| assert_metric.call(metric_name, sum) }
     end
   end