Support for new index templates

[consulthys]

In Elasticsearch 7.8, index templates have been refactored and this plugin does not yet support them, only the legacy templates.

When trying to send a refactored template, the error message is

Failed to install template. {:message=>"Got response code '400' contacting Elasticsearch at URL 'http://es01:9200/_template/my-logs'", :class=>"LogStash::Outputs::ElasticSearch::HttpClient::Pool::BadResponseCodeError", :backtrace=>["/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/manticore_adapter.rb:80:in perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/pool.rb:332:in perform_request_to_url'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/pool.rb:319:in block in perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/pool.rb:414:in with_connection'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/pool.rb:318:in perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client/pool.rb:326:in block in Pool'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client.rb:352:in template_put'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/http_client.rb:86:in template_install'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/template_manager.rb:28:in install'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/template_manager.rb:16:in install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/common.rb:205:in install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.5.1-java/lib/logstash/outputs/elasticsearch/common.rb:49:in block in setup_after_successful_connection'"]}

It would be nice to support the new index templating, i.e. the new _index_template endpoint in addition to the legacy _template endpoint.

To provide backward compatibility, it might be a good idea to provide a boolean flag template_legacy => true/false (default true)

[sastorsl]

Linking to the elasticsearch PR and issue where the legacy template API gets a deprecation warning.

elastic/elasticsearch#71307
elastic/elasticsearch#71309