Only initialize service URL once

[ycombinator]

What does this PR do?

It initializes the URL for the Logstash node stats API to be called by the logstash module when xpack.enabled: true is set exactly once.

Why is it important?

Before this fix, the ?vertices=true query parameter string was getting appended to the Logstash node stats API URL repeatedly, each time the logstash/node_stats's Fetch() method was invoked. This resulted in Logstash eventually returning HTTP 400 responses and Logstash monitoring data not being collected by the Logstash Metricbeat module.

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works. There are existing tests that verify correctness of collecting Logstash monitoring data. To test this particular bugfix, manual testing is OK IMO.

How to test this PR locally

1. Run Logstash with a simple pipeline.

   ./bin/logstash -e 'input { stdin {} }'
   

2. Enable the logstash-xpack module in Metricbeat.

   ./metricbeat modules enable logstash-xpack
   

3. Run Metricbeat.

   ./metricbeat -e
   

4. Open up Wireshark (or similar) and sniff for HTTP traffic on localhost going to port 9600 (the Logstash HTTP API port). Verify that you are seeing calls like GET /_node/stats?vertices=true every 10 seconds. Verify that the ?vertices=true query parameter string is not being appended in each iteration. So the first iteration should have exactly one ?vertices=true, as should the second, third, fourth, and so on.

Related issues

• Closes Metricbeat 7.5.2 silently stops collecting Logstash monitoring data #15974

[elasticmachine]

Pinging @elastic/stack-monitoring (Stack monitoring)

[ycombinator]

Travis CI is green and Jenkins CI failure is unrelated. Merging.