RFC: Instrumentable module using Prometheuszj-instrumentable

Using histograms, we can expose method timings. This is now done with
Influx, but we're moving away from this.

This commit itself is more of a PoC, and a request for comments rather
than a finished work.

[ci skip]

1 files changed, 58 insertions, 0 deletions

diff --git a/lib/gitlab/instrumentable.rb b/lib/gitlab/instrumentable.rb
new file mode 100644
index 00000000000..1d7e7b0f2df
--- /dev/null
+++ b/lib/gitlab/instrumentable.rb
@@ -0,0 +1,58 @@
+# Used to instrument methods in the GitLab codebase
+#
+# Creates a lightweight wrapper around the method pointed at by DSL method call
+# Under the hood this creates a Prometheus histogram, with at most 12 buckets to
+# keep that instance alife. Labels are not supported yet, as the number of labels
+# per histogram should remain limited, ideally to about 10. Also I didn't want
+# an extra DSL method like grape as that gets messy real quick.
+#
+# Usage:
+# class SomeClass
+#   include Gitlab::Instrumentable
+#
+#   instrument_method :some_method
+#
+#   # with custom buckets, max 12
+#   instrument_method :other_method, 20..30
+#   instrument_method :other_method, [1,3,5,8,12,30,60,120]
+# end
+require 'active_support/concern'
+
+module Gitlab
+  module Instrumentable
+    extend ActiveSupport::Concern
+
+    class_methods do
+      def instrument_method(method, buckets = nil)
+        @buckets = buckets&.to_a || ::Prometheus::Client::Histogram::DEFAULT_BUCKETS
+        raise 'Too many buckets, limit to 12' if @buckets.size > 12 && !Rails.env.production?
+
+        mod = Module.new do
+          define_method(method) do |*args|
+            return super(*args) if Rails.env.test?
+
+            start_time = Time.now.to_f
+            res = super(*args)
+            record_timing_since(start_time, __method__)
+
+            res
+          end
+
+          private
+
+          def record_timing_since(time, method)
+            class_name = self.class.name
+            @histogram ||=  Gitlab::Metrics.histogram(
+              "#{class_name.underscore}_duration_seconds",
+              "Execution time for #{class_name}##{method}",
+              {}, @buckets)
+
+            @histogram.observe({}, Time.now.to_f - time)
+          end
+        end
+
+        self.prepend(mod)
+      end
+    end
+  end
+end