Move Prometheus presentation logic to PrometheusText

+ Use NullMetrics to mock metrics when unused
+ Use method_missing in NullMetrics mocking
+ Update prometheus gem to version that correctly uses transitive dependencies
+ Ensure correct folders are used in Multiprocess prometheus client tests.
+ rename Sessions controller's metric

3 files changed, 45 insertions, 17 deletions

diff --git a/lib/gitlab/health_checks/prometheus_text.rb b/lib/gitlab/health_checks/prometheus_text.rb
new file mode 100644
index 00000000000..a01e6b2be1f
--- /dev/null
+++ b/lib/gitlab/health_checks/prometheus_text.rb
@@ -0,0 +1,38 @@
+module Gitlab::HealthChecks
+  class PrometheusText
+    def marshal(metrics)
+      metrics_with_type_declarations(metrics).join("\n")
+    end
+
+    private
+
+    def metrics_with_type_declarations(metrics)
+      type_declaration_added = {}
+
+      metrics.flat_map do |metric|
+        metric_lines = []
+
+        unless type_declaration_added.has_key?(metric.name)
+          type_declaration_added[metric.name] = true
+          metric_lines << metric_type_declaration(metric)
+        end
+
+        metric_lines << metric_text(metric)
+      end
+    end
+
+    def metric_type_declaration(metric)
+      "# TYPE #{metric.name} gauge"
+    end
+
+    def metric_text(metric)
+      labels = metric.labels&.map { |key, value| "#{key}=\"#{value}\"" }&.join(',') || ''
+
+      if labels.empty?
+        "#{metric.name} #{metric.value}"
+      else
+        "#{metric.name}{#{labels}} #{metric.value}"
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/metrics.rb b/lib/gitlab/metrics.rb
index a41cbd214a1..34f6b32f7da 100644
--- a/lib/gitlab/metrics.rb
+++ b/lib/gitlab/metrics.rb
@@ -73,7 +73,7 @@ module Gitlab
       if prometheus_metrics_enabled?
         registry.get(name)
       else
-        DummyMetric.new
+        NullMetric.new
       end
     end
 
diff --git a/lib/gitlab/metrics/dummy_metric.rb b/lib/gitlab/metrics/null_metric.rb
index d27bb83854a..1501cd38676 100644
--- a/lib/gitlab/metrics/dummy_metric.rb
+++ b/lib/gitlab/metrics/null_metric.rb
@@ -1,7 +1,12 @@
 module Gitlab
   module Metrics
     # Mocks ::Prometheus::Client::Metric and all derived metrics
-    class DummyMetric
+    class NullMetric
+      def method_missing(name, *args, &block)
+        nil
+      end
+
+      # these methods shouldn't be called when metrics are disabled
       def get(*args)
         raise NotImplementedError
       end
@@ -9,21 +14,6 @@ module Gitlab
       def values(*args)
         raise NotImplementedError
       end
-
-      # counter
-      def increment(*args)
-        # noop
-      end
-
-      # gauge
-      def set(*args)
-        # noop
-      end
-
-      # histogram / summary
-      def observe(*args)
-        # noop
-      end
     end
   end
 end