Benchmark API response parsing code

[ycombinator]

What does this PR do?

This PR makes the API response parsing code in the elasticsearch/index metricset benchmark-able, and adds a benchmark test for it. It is in preparation for #16538.

Why is it important?

To prove that the changes in #16538 will have a significant impact on memory usage while parsing the API response from Elasticsearch.

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

How to test this PR locally

cd $GOPATH/src/github.com/elastic/beats
go test -bench=. -benchmem ./metricbeat/module/elasticsearch/index/

Sample benchmark results

goos: darwin
goarch: amd64
pkg: github.com/elastic/beats/v7/metricbeat/module/elasticsearch/index
BenchmarkParseAPIResponse-8           25          44391131 ns/op        13464952 B/op     207889 allocs/op
PASS
ok      github.com/elastic/beats/v7/metricbeat/module/elasticsearch/index       1.585s

In terms of memory, the current API response parsing implementation consumed 13.46 MB.

Related issues

• Required for Reduce memory usage of elasticsearch.index metricset #16538

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

Pinging @elastic/stack-monitoring (Stack monitoring)

[ycombinator]

Travis CI is green and Jenkins CI failures are unrelated. Merging.