LogStash::Pipeline when collecting metrics in the pipeline global metric populates the different metrics

Examples:

https://travis-ci.org/elastic/logstash/jobs/254799125#L7518-L7544

https://logstash-ci.elastic.co/job/elastic+logstash+master+multijob-unix-compatibility/os=fedora/26/console

```
Failures:

1) LogStash::Pipeline when collecting metrics in the pipeline global metric populates the different metrics
Failure/Error: end.to be_truthy

expected: truthy value
got: false
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/handler.rb:13:in `block in handle_matcher'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/handler.rb:10:in `block in handle_matcher'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/handler.rb:9:in `handle_matcher'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/target.rb:30:in `block in to'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/target.rb:44:in `block in with_wait'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:28:in `with_wait'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/target.rb:44:in `with_wait'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait/target.rb:30:in `to'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/logstash-core/spec/logstash/pipeline_spec.rb:805:in `block in (root)'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `block in /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb'
# /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/lib/bootstrap/rspec.rb:13:in `'

Finished in 6 minutes 39 seconds (files took 12.53 seconds to load)
3051 examples, 1 failure, 3 pending

Failed examples:

rspec /var/lib/jenkins/workspace/elastic+logstash+master+multijob-unix-compatibility/os/fedora/logstash-core/spec/logstash/pipeline_spec.rb:816 # LogStash::Pipeline when collecting metrics in the pipeline global metric populates the different metrics

Randomized with seed 27058
```