[test failure: master] nodestats get failures on CentOS

https://logstash-ci.elastic.co/job/elastic+logstash+master+multijob-os-compatibility/os=centos/519/console

```
Finished in 5 minutes 15 seconds (files took 7.14 seconds to load)
2639 examples, 10 failures, 2 pending

Failed examples:

rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET /pipeline should set 'pipeline.events.queue_push_duration_in_millis' at '/pipeline' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET /pipeline should set 'pipeline.events.in' at '/pipeline' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET /pipeline should set 'pipeline.events.out' at '/pipeline' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET /pipeline should set 'pipeline.events.filtered' at '/pipeline' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET /pipeline should set 'pipeline.events.duration_in_millis' at '/pipeline' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET / should set 'pipeline.events.queue_push_duration_in_millis' at '/' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET / should set 'pipeline.events.filtered' at '/' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET / should set 'pipeline.events.duration_in_millis' at '/' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET / should set 'pipeline.events.in' at '/' to be a 'Numeric'
rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-os-compatibility/os/centos/spec/support/resource_dsl_methods.rb:63 # LogStash::Api::Modules::NodeStats GET / should set 'pipeline.events.out' at '/' to be a 'Numeric'

Randomized with seed 25492
```

Start with the linked CentOS compatibility CI log and the assertions at spec/support/resource_dsl_methods.rb:63 for LogStash::Api::Modules::NodeStats. Reproduce the failures for GET / and GET /pipeline, then trace why the listed pipeline event values are not Numeric on CentOS. Done means the ten reported assertions pass on the CentOS compatibility job.

Written by the indexing model from the issue text.