Api reloads count doesn't seems to be correctly set on the first try.

This is a message we have from our integration suite, but it doesn't generate any test failure, but logstash shouldn't crash and this values should always be present.

```
2017-04-24T19:25:33,200][INFO ][logstash.agent ] Successfully started Logstash API endpoint {:port=>9600}
[2017-04-24T19:25:33,282][ERROR][logstash.agent ] Internal API server error {:status=>500, :request_method=>"GET", :path_info=>"/_node/stats/pipeline", :query_string=>"", :http_version=>"HTTP/1.1", :http_accept=>nil, :error=>"Unexpected Internal Error", :class=>"LogStash::Instrument::MetricStore::MetricNotFound", :message=>"For path: pipelines. Map keys: [:reloads]", :backtrace=>["/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:225:in `get_recursively'", "org/jruby/RubyArray.java:1613:in `each'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:224:in `get_recursively'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:235:in `get_recursively'", "org/jruby/RubyArray.java:1613:in `each'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:224:in `get_recursively'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:95:in `get'", "org/jruby/ext/thread/Mutex.java:149:in `synchronize'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:94:in `get'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/instrument/metric_store.rb:108:in `get_shallow'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/api/service.rb:26:in `get_shallow'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/api/commands/stats.rb:49:in `pipeline'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/api/modules/node_stats.rb:48:in `pipeline_payload'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/logstash-core/lib/logstash/api/modules/node_stats.rb:12:in `HEAD /?:filter?'", "org/jruby/RubyMethod.java:116:in `call'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:1611:in `compile!'", "org/jruby/RubyProc.java:281:in `call'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:975:in `route!'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:994:in `route_eval'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:975:in `route!'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:1015:in `process_route'", "org/jruby/RubyKernel.java:1242:in `catch'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:1013:in `process_route'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:973:in `route!'", "org/jruby/RubyArray.java:1613:in `each'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-alpha1-SNAPSHOT/vendor/bundle/jruby/1.9/gems/sinatra-1.4.8/lib/sinatra/base.rb:972:in `route!'", "/home/travis/build/elastic/logstash/build/logstash-6.0.0-a