Make elasticsearch/index_summary metricset more defensive

[ycombinator]

Resolves #12487.

When there is absolutely no data in an Elasticsearch cluster, the elasticsearch/index_summary metricset, with xpack.enabled: true set, emits the following error in the Metricbeat log:

2019-06-10T03:00:47.439-0700    ERROR   [elasticsearch.index_summary]   index_summary/index_summary.go:92       failure applying stats schema: 8 errors: key `total.docs` not found; key `total.store` not found; key `total.indexing` not found; key `total.search` not found; key `primaries.docs` not found; key `primaries.store` not found; key `primaries.indexing` not found; key `primaries.search` not found

Having absolutely no data in the cluster is a "cold start" edge case that must be handled gracefully. This PR makes it so.

Concretely, this PR:

• Adds a guard clause to check if the GET _stats API response being used by the elasticsearch/index_summary metricset contains any stats for primary shards. If it doesn't, it doesn't try to parse the API response any further.
• Does a minor bit of refactoring, reusing the same schema structure in two places to avoid duplication of code.

Testing this PR

1. Make sure Elasticsearch API is accessible on http://localhost:9200.

2. Enable the elasticsearch-xpack Metricbeat module.

   ./metricbeat modules enable elasticsearch-xpack
   

3. Make sure there is no data in Elasticsearch. If there is, make sure nothing is indexing to it, and then run:

   curl -X DELETE 'http://localhost:9200/*'
   

4. Run Metricbeat.

   ./metricbeat -e
   

5. Assert that no errors are thrown in the Metricbeat log.

6. Assert that a .monitoring-es-*mb* index gets created in Elasticsearch.

   curl -s http://localhost:9200/_cat/indices/.monitoring*
   

[elasticmachine]

Pinging @elastic/stack-monitoring

[ruflin]

Code LGTM, did not test locally.

Do you want to add a changelog?

[cachedout]

I tested this locally and it behaves as described. 👍

[ycombinator]

Added CHANGELOG entry. Will merge when CI goes green again.

[ycombinator]

jenkins, test this