Add component to comment with the database role

With multiple databases in Rails 6, it can be difficult to tell from
logging if your database query is being made against the leader or
follower database. This change adds a new component, `db_role`, which
can be used with the database host and name to annotate the query with
the current ActiveRecord role.

lib/marginalia/comment.rb

@@ -162,6 +162,11 @@ def self.database
         end
       end
 
+      def self.db_role
+        return if marginalia_adapter.pool.nil?
+        ActiveRecord::Base.current_role
+      end
+
       if Gem::Version.new(ActiveRecord::VERSION::STRING) < Gem::Version.new('6.1')
         def self.connection_config
           return if marginalia_adapter.pool.nil?

test/query_comments_test.rb

@@ -9,6 +9,10 @@ def pool_db_config?
   Gem::Version.new(ActiveRecord::VERSION::STRING) >= Gem::Version.new('6.1')
 end
 
+def database_role_available?
+  Gem::Version.new(ActiveRecord::VERSION::STRING) >= Gem::Version.new('6.0.0')
+end
+
 require "minitest/autorun"
 require "mocha/minitest"
 require 'logger'
@@ -259,6 +263,14 @@ def test_socket
     end
   end
 
+  if database_role_available?
+    def test_db_role
+      Marginalia::Comment.components = [:db_role]
+      API::V1::PostsController.action(:driver_only).call(@env)
+      assert_match %r{/\*db_role:writing}, @queries.first
+    end
+  end
+
   def test_request_id
     @env["action_dispatch.request_id"] = "some-uuid"
     Marginalia::Comment.components = [:request_id]