1 files changed, 129 insertions, 0 deletions
diff --git a/lib/gitlab/metrics/methods.rb b/lib/gitlab/metrics/methods.rb
new file mode 100644
index 00000000000..cd7c1e507f7
--- /dev/null
+++ b/lib/gitlab/metrics/methods.rb
@@ -0,0 +1,129 @@
+# rubocop:disable Style/ClassVars
+
+module Gitlab
+  module Metrics
+    module Methods
+      extend ActiveSupport::Concern
+
+      included do
+        @@_metric_provider_mutex ||= Mutex.new
+        @@_metrics_provider_cache = {}
+      end
+
+      class_methods do
+        def reload_metric!(name)
+          @@_metrics_provider_cache.delete(name)
+        end
+
+        private
+
+        def define_metric(type, name, opts = {}, &block)
+          if respond_to?(name)
+            raise ArgumentError, "method #{name} already exists"
+          end
+
+          define_singleton_method(name) do
+            # inlining fetch_metric method to avoid method call overhead when instrumenting hot spots
+            @@_metrics_provider_cache[name] || init_metric(type, name, opts, &block)
+          end
+        end
+
+        def fetch_metric(type, name, opts = {}, &block)
+          @@_metrics_provider_cache[name] || init_metric(type, name, opts, &block)
+        end
+
+        def init_metric(type, name, opts = {}, &block)
+          options = MetricOptions.new(opts)
+          options.evaluate(&block)
+
+          if disabled_by_feature(options)
+            synchronized_cache_fill(name) { NullMetric.instance }
+          else
+            synchronized_cache_fill(name) { build_metric!(type, name, options) }
+          end
+        end
+
+        def synchronized_cache_fill(key)
+          @@_metric_provider_mutex.synchronize do
+            @@_metrics_provider_cache[key] ||= yield
+          end
+        end
+
+        def disabled_by_feature(options)
+          options.with_feature && !Feature.get(options.with_feature).enabled?
+        end
+
+        def build_metric!(type, name, options)
+          case type
+          when :gauge
+            Gitlab::Metrics.gauge(name, options.docstring, options.base_labels, options.multiprocess_mode)
+          when :counter
+            Gitlab::Metrics.counter(name, options.docstring, options.base_labels)
+          when :histogram
+            Gitlab::Metrics.histogram(name, options.docstring, options.base_labels, options.buckets)
+          when :summary
+            raise NotImplementedError, "summary metrics are not currently supported"
+          else
+            raise ArgumentError, "uknown metric type #{type}"
+          end
+        end
+
+        # Fetch and/or initialize counter metric
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def fetch_counter(name, opts = {}, &block)
+          fetch_metric(:counter, name, opts, &block)
+        end
+
+        # Fetch and/or initialize gauge metric
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def fetch_gauge(name, opts = {}, &block)
+          fetch_metric(:gauge, name, opts, &block)
+        end
+
+        # Fetch and/or initialize histogram metric
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def fetch_histogram(name, opts = {}, &block)
+          fetch_metric(:histogram, name, opts, &block)
+        end
+
+        # Fetch and/or initialize summary metric
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def fetch_summary(name, opts = {}, &block)
+          fetch_metric(:summary, name, opts, &block)
+        end
+
+        # Define metric accessor method for a Counter
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def define_counter(name, opts = {}, &block)
+          define_metric(:counter, name, opts, &block)
+        end
+
+        # Define metric accessor method for a Gauge
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def define_gauge(name, opts = {}, &block)
+          define_metric(:gauge, name, opts, &block)
+        end
+
+        # Define metric accessor method for a Histogram
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def define_histogram(name, opts = {}, &block)
+          define_metric(:histogram, name, opts, &block)
+        end
+
+        # Define metric accessor method for a Summary
+        # @param [Symbol] name
+        # @param [Hash] opts
+        def define_summary(name, opts = {}, &block)
+          define_metric(:summary, name, opts, &block)
+        end
+      end
+    end
+  end
+end