[fluentd-elasticsearch] errros when upgrading from 11.14.0 to 11.15.0

[TheMeier]

Describe the bug
When upgrading the helm chart from 11.14.0 to 11.15.0 i get the following error:

fluentd-elasticsearch-gn4nx fluentd-elasticsearch {"time":"2021-11-03 10:24:25 +0000","level":"error","message":"unexpected error error_class=NoMethodError error=\"undefined method `host_unreachable_exceptions' for #<Elasticsearch::Transport::Client:0x00007fbb7ed98cf8>\""}
fluentd-elasticsearch-gn4nx fluentd-elasticsearch {"time":"2021-11-03 10:24:25 +0000","level":"error","message":"/usr/local/bundle/gems/fluent-plugin-elasticsearch-5.0.5/lib/fluent/plugin/elasticsearch_index_template.rb:41:in `rescue in retry_operate'\n/usr/local/bundle/gems/fluent-plugin-elasticsearch-5.0.5/lib/fluent/plugin/elasticsearch_index_template.rb:39:in `retry_operate'\n/usr/local/bundle/gems/fluent-plugin-elasticsearch-5.0.5/lib/fluent/plugin/out_elasticsearch.rb:487:in `handle_last_seen_es_major_version'\n/usr/local/bundle/gems/fluent-plugin-elasticsearch-5.0.5/lib/fluent/plugin/out_elasticsearch.rb:339:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/plugin.rb:178:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/agent.rb:132:in `add_match'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/agent.rb:74:in `block in configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/agent.rb:64:in `each'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/agent.rb:64:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/label.rb:31:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:143:in `block in configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:143:in `each'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:143:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/engine.rb:105:in `configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/engine.rb:80:in `run_configure'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/supervisor.rb:714:in `block in run_worker'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/supervisor.rb:966:in `main_process'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/supervisor.rb:706:in `run_worker'\n/usr/local/bundle/gems/fluentd-1.13.3/lib/fluent/command/fluentd.rb:364:in `<top (required)>'\n/usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:72:in `require'\n/usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:72:in `require'\n/usr/local/bundle/gems/fluentd-1.13.3/bin/fluentd:15:in `<top (required)>'\n/usr/local/bundle/bin/fluentd:23:in `load'\n/usr/local/bundle/bin/fluentd:23:in `<main>'"}

Version of Helm and Kubernetes:

Helm Version:

$ helm version
version.BuildInfo{Version:"v3.6.3", GitCommit:"d506314abfb5d21419df8c7e7e68012379db2354", GitTreeState:"clean", GoVersion:"go1.16.5"}

Kubernetes Version:

$ kubectl version
Client Version: version.Info{Major:"1", Minor:"19", GitVersion:"v1.19.14", GitCommit:"0fd2b5afdfe3134d6e1531365fdb37dd11f54d1c", GitTreeState:"clean", BuildDate:"2021-08-11T18:07:41Z", GoVersion:"go1.15.15", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"19", GitVersion:"v1.19.11", GitCommit:"c6a2f08fc4378c5381dd948d9ad9d1080e3e6b33", GitTreeState:"clean", BuildDate:"2021-05-12T12:19:22Z", GoVersion:"go1.15.12", Compiler:"gc", Platform:"linux/amd64"}

Which version of the chart:
11.15.0

What happened:
After upgrading the pods go into CrashLoopBackOff the logs show the error above

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

<~--
This could be something like:

values.yaml (only put values which differ from the defaults)

elasticsearch:
  suppressTypeName: true
  scheme: https
  logLevel: info
  auth:
    enabled: true
    user: user
    password: password
  hosts:
    - "opendistro-ingest-1:9200"
    - "opendistro-ingest-2:9200"
  logstash:
    prefix: k8s-dev
tolerations:
  - key: node-role.kubernetes.io/master
    operator: Exists
    effect: NoSchedule
fluentdLogFormat: json
prometheusRule:
  enabled: true
  prometheusNamespace: core-logging
serviceMonitor
  enabled: true
  jobLabel fluentd

Anything else we need to know:
After a rollback to 11.14.0 everything works as expected. Target elasticsearch is elasticsearch-oss-7.10.2-1.x86_64 from opendistroforelasticsearch-1.13.2-1.x86_64.
uken/fluent-plugin-elasticsearch#912 mentions this could be due to 7.14 client library from elasticsearch.

[s7an-it]

Getting it with latest chart and 7.15.2 ES.