[Client] Log warnings instead of using Kernel.warn

Closes #1508

Author: picandocodigo

elasticsearch-transport/lib/elasticsearch/transport/transport/base.rb

@@ -348,7 +348,7 @@ def perform_request(method, path, params = {}, body = nil, headers = nil, opts =
           end
 
           __trace(method, path, params, connection_headers(connection), body, url, response, nil, 'N/A', duration) if tracer
-          warnings(response.headers['warning']) if response.headers&.[]('warning')
+          log_warn(response.headers['warning']) if response.headers&.[]('warning')
           Response.new response.status, json || response.body, response.headers
         ensure
           @last_request_at = Time.now
@@ -423,10 +423,6 @@ def user_agent_header(client)
           end
         end
 
-        def warnings(warning)
-          warn("warning: #{warning}")
-        end
-
         def connection_headers(connection)
           if defined?(Elasticsearch::Transport::Transport::HTTP::Manticore) && self.class == Elasticsearch::Transport::Transport::HTTP::Manticore
             @request_options[:headers]

elasticsearch-transport/spec/elasticsearch/transport/client_spec.rb

@@ -1466,28 +1466,19 @@
     end
 
     context 'when Elasticsearch response includes a warning header' do
+      let(:logger) { double('logger', warn: '', warn?: '', info?: '', info: '', debug?: '', debug: '') }
       let(:client) do
-        Elasticsearch::Transport::Client.new(hosts: hosts)
+        Elasticsearch::Transport::Client.new(hosts: hosts, logger: logger)
       end
 
       let(:warning) { 'Elasticsearch warning: "deprecation warning"' }
 
       it 'prints a warning' do
-        allow_any_instance_of(Elasticsearch::Transport::Transport::Response).to receive(:headers) do
-          { 'warning' => warning }
-        end
-
-        begin
-          stderr      = $stderr
-          fake_stderr = StringIO.new
-          $stderr     = fake_stderr
-
-          client.perform_request('GET', '/')
-          fake_stderr.rewind
-          expect(fake_stderr.string).to eq("warning: #{warning}\n")
-        ensure
-          $stderr = stderr
+        expect_any_instance_of(Faraday::Connection).to receive(:run_request) do
+          Elasticsearch::Transport::Transport::Response.new(200, {}, { 'warning' => warning })
         end
+        client.perform_request('GET', '/')
+        expect(logger).to have_received(:warn).with(warning)
       end
     end