Monitor GraphQL with Prometheus

Extends graphql's platform tracing class to observe duration
of graphql methods.

In graphql 1.8.11 is added prometheus class but it's not very useful
for us because it uses prometheus_exporter to export results.

app/graphql/gitlab_schema.rb

@@ -5,6 +5,7 @@ class GitlabSchema < GraphQL::Schema
   use Gitlab::Graphql::Authorize
   use Gitlab::Graphql::Present
   use Gitlab::Graphql::Connections
+  use Gitlab::Graphql::Tracing
   query(Types::QueryType)

changelogs/unreleased/graphql-prometheus.yml

+---
+title: Added prometheus monitoring to GraphQL
+merge_request:
+author:
+type: added

lib/gitlab/graphql/tracing.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Graphql
+    class Tracing < GraphQL::Tracing::PlatformTracing
+      self.platform_keys = {
+        'lex' => 'graphql.lex',
+        'parse' => 'graphql.parse',
+        'validate' => 'graphql.validate',
+        'analyze_query' => 'graphql.analyze',
+        'analyze_multiplex' => 'graphql.analyze',
+        'execute_multiplex' => 'graphql.execute',
+        'execute_query' => 'graphql.execute',
+        'execute_query_lazy' => 'graphql.execute',
+        'execute_field' => 'graphql.execute',
+        'execute_field_lazy' => 'graphql.execute'
+      }
+
+      def platform_field_key(type, field)
+        "#{type.name}.#{field.name}"
+      end
+
+      def platform_trace(platform_key, key, data, &block)
+        start = Gitlab::Metrics::System.monotonic_time
+
+        yield
+      ensure
+        duration = Gitlab::Metrics::System.monotonic_time - start
+
+        graphql_duration_seconds.observe({ platform_key: platform_key, key: key }, duration)
+      end
+
+      private
+
+      def graphql_duration_seconds
+        @graphql_duration_seconds ||= Gitlab::Metrics.histogram(
+          :graphql_duration_seconds,
+          'GraphQL execution time'
+        )
+      end
+    end
+  end
+end

spec/lib/gitlab/graphql/tracing_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Graphql::Tracing do
+  let(:graphql_duration_seconds_histogram) { double('Gitlab::Metrics::NullMetric') }
+
+  it 'updates graphql histogram with expected labels' do
+    query = 'query { users { id } }'
+    tracer = described_class.new
+
+    allow(tracer)
+      .to receive(:graphql_duration_seconds)
+      .and_return(graphql_duration_seconds_histogram)
+
+    expect_metric('graphql.lex', 'lex')
+    expect_metric('graphql.parse', 'parse')
+    expect_metric('graphql.validate', 'validate')
+    expect_metric('graphql.analyze', 'analyze_multiplex')
+    expect_metric('graphql.execute', 'execute_query_lazy')
+    expect_metric('graphql.execute', 'execute_multiplex')
+
+    GitlabSchema.execute(query, context: { tracers: [tracer] })
+  end
+
+  private
+
+  def expect_metric(platform_key, key)
+    expect(graphql_duration_seconds_histogram)
+      .to receive(:observe)
+      .with({ platform_key: platform_key, key: key }, be > 0.0)
+  end
+end