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.

4 files changed, 82 insertions, 0 deletions

diff --git a/app/graphql/gitlab_schema.rb b/app/graphql/gitlab_schema.rb
index 06d26309b5b..ecc34eacc7d 100644
--- a/app/graphql/gitlab_schema.rb
+++ b/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)
 
diff --git a/changelogs/unreleased/graphql-prometheus.yml b/changelogs/unreleased/graphql-prometheus.yml
new file mode 100644
index 00000000000..180577f3aec
--- /dev/null
+++ b/changelogs/unreleased/graphql-prometheus.yml
@@ -0,0 +1,5 @@
+---
+title: Added prometheus monitoring to GraphQL
+merge_request:
+author:
+type: added
diff --git a/lib/gitlab/graphql/tracing.rb b/lib/gitlab/graphql/tracing.rb
new file mode 100644
index 00000000000..6b505e4262b
--- /dev/null
+++ b/lib/gitlab/graphql/tracing.rb
@@ -0,0 +1,43 @@
+# 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
diff --git a/spec/lib/gitlab/graphql/tracing_spec.rb b/spec/lib/gitlab/graphql/tracing_spec.rb
new file mode 100644
index 00000000000..7300a9a572e
--- /dev/null
+++ b/spec/lib/gitlab/graphql/tracing_spec.rb
@@ -0,0 +1,33 @@
+# 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