Adds prometheus metrics collection and endpoint

Gemfile

@@ -12,6 +12,7 @@ gem 'jwt'
 gem 'lograge'
 gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.2'
 gem 'opensearch-ruby'
+gem 'prometheus-client'
 gem 'puma'
 gem 'rack-attack'
 gem 'rack-cors'

Gemfile.lock

@@ -248,6 +248,7 @@ GEM
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     pg (1.5.4)
+    prometheus-client (4.2.2)
     psych (5.1.2)
       stringio
     public_suffix (5.0.4)
@@ -433,6 +434,7 @@ DEPENDENCIES
   mitlibraries-theme!
   opensearch-ruby
   pg
+  prometheus-client
   puma
   rack-attack
   rack-cors

README.md

@@ -185,6 +185,7 @@ locally.
 - `PREFERRED_DOMAIN` - set this to the domain you would like to to use. Any
   other requests that come to the app will redirect to the root of this domain.
   This is useful to prevent access to herokuapp.com domains.
+- `PROMETHEUS` - If present, enables the Prometheus metrics endpoint and the feature flag to capture metrics
 - `REQUESTS_PER_PERIOD` - requests allowed before throttling. Default is 100.
 - `REQUEST_PERIOD` - number of minutes for the period in `REQUESTS_PER_PERIOD`.
   Default is 1.

app/controllers/graphql_controller.rb

@@ -10,9 +10,12 @@ def execute
       # current_user: current_user,
       tracers: [request_tracer]
     }
-    result = TimdexSchema.execute(query, variables: variables,
-                                         context: context,
-                                         operation_name: operation_name)
+    result = TimdexSchema.execute(query, variables:,
+                                         context:,
+                                         operation_name:)
+
+    Timdex::GraphqlQueriesTotal.increment if Flipflop.enabled?(:prometheus)
+
     render json: result
   rescue StandardError => e
     raise e unless Rails.env.development?

app/graphql/timdex_field_usage_analyzer.rb

@@ -8,6 +8,27 @@ def result
     Rails.logger.info("GraphQL used fields: #{@used_fields.to_a}")
     Rails.logger.info("GraphQL used deprecated fields: #{@used_deprecated_fields.to_a}")
     Rails.logger.info("GraphQL used deprecated arguments: #{@used_deprecated_arguments.to_a}")
+
+    if Flipflop.enabled?(:prometheus)
+      # Increment deprecated field usage counter per field
+      @used_deprecated_fields.each do |field|
+        Timdex::GraphqlDeprecatedFieldUsage.increment(labels: { field: })
+      end
+
+      # Increment field usage counter per field
+      @used_fields.each do |field|
+        next if @used_deprecated_fields.include?(field)
+        next if field.start_with?('_')
+
+        Timdex::GraphqlFieldUsage.increment(labels: { field: })
+      end
+
+      # Increment deprecated argument counter per argument
+      @used_deprecated_arguments.each do |field|
+        Timdex::GraphqlDeprecatedArguments.increment(labels: { field: })
+      end
+    end
+
     {
       used_fields: @used_fields.to_a,
       used_deprecated_fields: @used_deprecated_fields.to_a,

config.ru

@@ -4,3 +4,13 @@ require_relative 'config/environment'
 
 run Rails.application
 Rails.application.load_server
+
+if ENV.fetch('PROMETHEUS', false).present?
+  require 'rack'
+  require 'prometheus/middleware/collector'
+  require 'prometheus/middleware/exporter'
+
+  use Rack::Deflater
+  use Prometheus::Middleware::Collector
+  use Prometheus::Middleware::Exporter
+end

config/features.rb

@@ -2,4 +2,8 @@
   # Strategies will be used in the order listed here.
   strategy :session
   strategy :default
+
+ feature :prometheus,
+    default: ENV.fetch('PROMETHEUS', false),
+    description: "Enable prometheus metrics endpoint"
 end

config/initializers/prometheus_metrics.rb

@@ -0,0 +1,14 @@
+# flipflip features are available to use in initializers so we cheat a bit here.
+return unless ENV.fetch('PROMETHEUS', false).present?
+
+require 'prometheus/client'
+require 'prometheus/client/push'
+
+prometheus = Prometheus::Client.registry
+Timdex::GraphqlQueriesTotal = prometheus.counter(:graphql_queries_total, docstring: 'A counter of GraphQL requests made')
+
+Timdex::GraphqlFieldUsage = prometheus.counter(:graphql_field_usage, docstring: 'A counter of GraphQL fields requested', labels: [:service, :field], preset_labels: { service: "timdex-api" })
+
+Timdex::GraphqlDeprecatedFieldUsage = prometheus.counter(:graphql_deprecated_field_usage, docstring: 'A counter of deprecated GraphQL fields requested', labels: [:service, :field], preset_labels: { service: "timdex-api" })
+
+Timdex::GraphqlDeprecatedArguments = prometheus.counter(:graphql_deprecated_argument_usage, docstring: 'A counter of deprecated GraphQL arguments requested', labels: [:service, :field], preset_labels: { service: "timdex-api" })