Add info about prometheus buckets

+ fix cpu time

lib/gitlab/metrics/influx_db.rb

@@ -11,6 +11,8 @@ module Gitlab
         settings[:enabled] || false
       end
 
+      # Prometheus histogram buckets used for arbitrary code measurements
+      EXECUTION_MEASUREMENT_BUCKETS = [0.001, 0.002, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1].freeze
       RAILS_ROOT = Rails.root.to_s
       METRICS_ROOT = Rails.root.join('lib', 'gitlab', 'metrics').to_s
       PATH_REGEX = /^#{RAILS_ROOT}\/?/
@@ -99,22 +101,23 @@ module Gitlab
         cpu_stop = System.cpu_time
         real_stop = Time.now.to_f
 
-        real_time = (real_stop - real_start) * 1000.0
+        real_time = (real_stop - real_start)
         cpu_time = cpu_stop - cpu_start
 
         Gitlab::Metrics.histogram("gitlab_#{name}_real_duration_seconds".to_sym,
                                   "Measure #{name}",
                                   Transaction::BASE_LABELS,
-                                  [0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2])
-          .observe(trans.labels, real_time / 1000.0)
+                                  EXECUTION_MEASUREMENT_BUCKETS)
+          .observe(trans.labels, real_time)
 
         Gitlab::Metrics.histogram("gitlab_#{name}_cpu_duration_seconds".to_sym,
                                   "Measure #{name}",
                                   Transaction::BASE_LABELS,
-                                  [0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2])
+                                  EXECUTION_MEASUREMENT_BUCKETS)
           .observe(trans.labels, cpu_time / 1000.0)
 
-        trans.increment("#{name}_real_time", real_time, false)
+        # InfluxDB stores the _real_time time values as milliseconds
+        trans.increment("#{name}_real_time", real_time * 1000, false)
         trans.increment("#{name}_cpu_time", cpu_time, false)
         trans.increment("#{name}_call_count", 1, false)
 

lib/gitlab/metrics/method_call.rb

@@ -46,8 +46,7 @@ module Gitlab
         cpu_time = System.cpu_time - start_cpu
 
         @real_time += real_time
-
-        @cpu_time += System.cpu_time - start_cpu
+        @cpu_time += cpu_time
         @call_count += 1
 
         self.class.call_real_duration_histogram.observe(@transaction.labels.merge(labels), real_time / 1000.0)