Add JSON logger in `log/api_json.log` for Grape API endpoints

Closes #36189

Gemfile

@@ -407,3 +407,4 @@ gem 'flipper-active_record', '~> 0.10.2'
 # Structured logging
 gem 'lograge', '~> 0.5'
+gem 'grape_logging', '~> 1.6'

Gemfile.lock

@@ -355,6 +355,8 @@ GEM
       activesupport
       grape (>= 0.16.0)
       rake
+    grape_logging (1.6.0)
+      grape
     grpc (1.4.5)
       google-protobuf (~> 3.1)
       googleauth (~> 0.5.1)
@@ -1034,6 +1036,7 @@ DEPENDENCIES
   grape (~> 1.0)
   grape-entity (~> 0.6.0)
   grape-route-helpers (~> 2.1.0)
+  grape_logging (~> 1.6)
   haml_lint (~> 0.26.0)
   hamlit (~> 2.6.1)
   hashie-forbidden_attributes

changelogs/unreleased/sh-add-grape-logging.yml

+---
+title: Add JSON logger in `log/api_json.log` for Grape API endpoints
+merge_request:
+author:
+type: added

lib/api/api.rb

@@ -2,6 +2,15 @@ module API
   class API < Grape::API
     include APIGuard
+    LOG_FILENAME = Rails.root.join("log", "api_json.log")
+
+    use GrapeLogging::Middleware::RequestLogger,
+        logger: ::Gitlab::ApiLogger.new(LOG_FILENAME),
+        formatter: GrapeLogging::Formatters::Json.new,
+        include: [ GrapeLogging::Loggers::Response.new,
+                   GrapeLogging::Loggers::FilterParameters.new,
+                   GrapeLogging::Loggers::ClientEnv.new ]
+
     allow_access_with_scope :api
     prefix :api

lib/gitlab/api_logger.rb

+module Gitlab
+  class ApiLogger < ::Logger
+
+    def format_message(severity, timestamp, progname, message)
+      super + "\n"
+    end
+  end
+end