[receiver/elasticsearch] Enable more index metrics by default

[crobert-1]

Description:

This enables the following metrics by default:

elasticsearch.index.documents
elasticsearch.index.operations.merge.current
elasticsearch.index.segments.count

Link to tracking Issue:
Resolves #34396

Testing:
Tests have been updated to account for these metrics being enabled by default. One thing to note, some tests were manually enabling these metrics. This is no longer necessary, but I've left as-is since it doesn't do any harm, and can be helpful to ensure tests act the way we want. I can update though if reviewers prefer.

Testing side note: For some reason, when running integration tests locally, I had to add a time.Sleep call before scraping when the docker container was being spun up. Some metrics and resource attributes weren't populated fully without the delay, and caused a mismatch with the GitHub CI's results that are generated without the sleep call.