Add warnings to setup.template.overwrite

[Leaf-Lin]

Motivation/summary

Similar to elastic/beats#22357, setup.template.overwrite could potentially overload Elasticsearch with too many update requests.
On Elasticsearch side, it will be addressed in newer versions by introducing no-op updates which will be available from 7.11+ elastic/elasticsearch#64493

See also elastic/elasticsearch#57662

Thought having some warnings in the APM setting here should prevent users unwittingly leave template auto-creation on across large number of APM.

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:

• documentation
• logging (add log lines, choose appropriate log selector, etc.)
• metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

[apmmachine]

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Build stats

• Build Cause: Pull request #4463 opened

• Start Time: 2020-11-27T04:15:51.170+0000

• Duration: 41 min 32 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4754
Skipped	143
Total	4897

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• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

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• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

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• Description: ./.ci/scripts/sync.sh

[axw]

Thanks @Leaf-Lin! I imagine that because APM Server is (currently) mostly deployed in smaller numbers, it wouldn't be such an issue. Once we move to running under Elastic Agent, on the edge, that would change; but we'll also be moving template management to Fleet then. Anyway, doesn't hurt to be cautious here.

@bmorelli25 I'm not sure how/when we normally update these copied-from-beats docs, so I'll leave it to you.

[bmorelli25]

Thanks, @Leaf-Lin!

This documentation is shared by both Beats and APM Server, with Beats being the source of truth. It looks like Naomi added this note to the Beats documentation recently, but we haven't updated the file on the APM side. Because APM Server programmatically updates documentation from Beats, committing this PR would only be temporary -- the contents would be overwritten on our next update.

I've opened elastic/beats#22804 to address a few typos in the original PR. Once that is merged, I'll pull the changes into APM Server. In the meantime, I'll close this PR. Thanks again :)