Convert license struct to MapStr

[ycombinator]

Currently when the elasticsearch module prepares a cluster_stats document to index into .monitoring-es-*, it marshals any values representing ms-since-epoch into scientific notation, e.g. 1.572634502804E12. Elasticsearch needs these values to be preserved as regular integers, e.g. 1572634502804.

Turns out this only happens when marshaling a struct containing such values. Marshaling a common.MapStr marshals the values as expected.

Looking through the elasticsearch module code for the cluster_stats metricset, the only struct that was being used in preparing the document for ES was the License struct. This PR adds a toMapStr() method to this struct and uses its results in the document instead.

Testing this PR

1. Start up Elasticsearch.

2. Build Metricbeat with this PR.

   cd $GOPATH/src/github.com/elastic/beats/metricbeat
   mage build
   

3. Enable the elasticsearch-xpack Metricbeat module.

 ./metricbeat modules enable elasticsearch-xpack

4. Run Metricbeat

   ./metricbeat -e
   

5. Verify that type:cluster_stats documents in .monitoring-es-* contain regular integer values for ms-since-epoch fields under the license field.

   GET .monitoring-es-*/_search?q=type:cluster_stats&sort=timestamp:desc&filter_path=hits.hits._source.license*
   

[elasticmachine]

Pinging @elastic/stack-monitoring (Stack monitoring)

[kaiyan-sheng]

The code looks good to me. I tried to test it but I think it requires the platinum ES license. Any unit test you can add for making sure ms-since-epoch values are marshaled correctly?

[jakelandis]

Changes looks good (but I am not terribly familiar with this code base or language). Thanks for fixing this !

[ycombinator]

The code looks good to me. I tried to test it but I think it requires the platinum ES license.

Trial license works as well.

Any unit test you can add for making sure ms-since-epoch values are marshaled correctly?

I've been trying to do this for the past couple hours, actually. From what I can tell the marshalling from int to double is happening at a lower level, perhaps in libbeat. So I'm not sure a simple unit test in the elasticsearch Metricbeat module would do the trick here. Since the symptom of the problem shows up in an Elasticsearch document, I might have to resort to a system test here.

[ycombinator]

@kaiyan-sheng I've added a system test per my previous comment. Please re-review when you get a chance. Thanks!

[EDIT] FWIW, I ran the same system test on master and it fails (as expected)!

[kaiyan-sheng]

LGTM, thanks for adding the test.

[cachedout]

Tested and works as expected. Code looks good as well. Nice work!