<Assume_role_credentials> throw MissingCredentialsError

I am running fluentd in a Kubernetes cluster. When configuring the `s3_out` plugin, instead of providing the `access_key` and the `secret_key`, I used the ``. My s3 configuration looks like this -

```

@type s3
@log_level debug

role_arn arn:aws:iam::1111111111111:role/fluentd-test-role
role_session_name Admin

s3_bucket "sumo-log"
s3_region "us-west-2"
path logs/
time_slice_format %Y%m%d%H%M
utc
auto_create_bucket true

```

However, I get the following error
```
2018-10-12 18:50:25 +0000 [info]: #0 starting fluentd worker pid=30 ppid=1 worker=0
2018-10-12 18:50:25 +0000 [error]: #0 unexpected error error_class=Aws::Errors::MissingCredentialsError error="unable to sign request without credentials set"
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/signature_v4.rb:72:in `sign_request'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/signature_v4.rb:112:in `apply_signature'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/signature_v4.rb:65:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/helpful_socket_errors.rb:10:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/retry_errors.rb:138:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/query/handler.rb:28:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/user_agent.rb:13:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/seahorse/client/plugins/endpoint.rb:45:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/param_validator.rb:24:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/seahorse/client/plugins/raise_response_errors.rb:14:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/jsonvalue_converter.rb:20:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/idempotency_token.rb:17:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/param_converter.rb:24:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/plugins/response_paging.rb:10:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/seahorse/client/plugins/response_target.rb:23:in `call'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/seahorse/client/request.rb:70:in `send_request'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-sts/client.rb:486:in `assume_role'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/assume_role_credentials.rb:49:in `refresh'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/refreshing_credentials.rb:20:in `initialize'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/aws-sdk-core-3.30.0/lib/aws-sdk-core/assume_role_credentials.rb:40:in `initialize'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluent-plugin-s3-1.1.4/lib/fluent/plugin/out_s3.rb:426:in `new'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluent-plugin-s3-1.1.4/lib/fluent/plugin/out_s3.rb:426:in `setup_credentials'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluent-plugin-s3-1.1.4/lib/fluent/plugin/out_s3.rb:198:in `start'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:165:in `block in start'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:154:in `block (2 levels) in lifecycle'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:153:in `each'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:153:in `block in lifecycle'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:140:in `each'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:140:in `lifecycle'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/root_agent.rb:164:in `start'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/engine.rb:274:in `start'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/engine.rb:219:in `run'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/supervisor.rb:774:in `run_engine'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/supervisor.rb:523:in `block in run_worker'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/supervisor.rb:699:in `main_process'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/supervisor.rb:518:in `run_worker'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/lib/fluent/command/fluentd.rb:316:in `'
2018-10-12 18:50:25 +0000 [error]: #0 /usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
2018-10-12 18:50:25 +0000 [error]: #0 /usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
2018-10-12 18:50:25 +0000 [error]: #0 /var/lib/gems/2.3.0/gems/fluentd-1.1.3/bin/fluentd:8:in `'
2018-10-12 18:50:25 +0000 [error]: #0 /usr/local/bin/fluentd:22:in `load'
2018-10-12 18:50:25 +0000 [error]: #0 /usr/local/bin/fluentd:22:in `'
2018-10-12 18:50:25 +0000 [error]: #0 unexpected error error_class=Aws::Errors::MissingCredentialsError error="unable to sign request without credentials set"
2018-10-12 18:50:25 +0000 [error]: #0 suppressed same stacktrace
2018-10-12 18:50:25 +0000 [info]: Worker 0 finished unexpectedly with status 1
```

I should be able to access AWS without providing the `access key and secret key`.

Can anybody provide more details about it?