fix: ensure ES index names are lowercase for ILM

[simitt]

Motivation/summary

Ensure ILM custom index suffixes are lowercased to ensure templates and indices in ES can be created.

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)

How to test these changes

configure APM Server to setup ILM with index-suffixes that are uppercase, start APM Server and ensure templates and indices are created with the index-suffix lowercased.

Related issues

fixes #4321

[codecov-io]

Codecov Report

Merging #4322 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4322   +/-   ##
=======================================
  Coverage   79.67%   79.67%           
=======================================
  Files         161      162    +1     
  Lines        8983     8994   +11     
=======================================
+ Hits         7157     7166    +9     
- Misses       1826     1828    +2     

Impacted Files	Coverage Δ
idxmgmt/ilm/config.go	82.40% <100.00%> (ø)
log/ratelimit.go	100.00% <100.00%> (ø)
model/modeldecoder/rumv3/decoder.go	94.97% <100.00%> (ø)
model/modeldecoder/v2/decoder.go	95.90% <100.00%> (+0.01%)	⬆️
...ack/apm-server/aggregation/txmetrics/aggregator.go	92.08% <100.00%> (+0.20%)	⬆️
beater/jaeger/common.go	75.00% <0.00%> (-7.15%)	⬇️

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #4322 updated]

• Start Time: 2020-10-15T10:19:57.429+0000

• Duration: 44 min 12 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4293
Skipped	145
Total	4438

Steps errors

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• Name: Run Window tests

  • Description:

  • Duration: 9 min 25 sec

  • Start Time: 2020-10-15T10:37:37.364+0000

  • log

• Name: Compress

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

  • Duration: 0 min 0 sec

  • Start Time: 2020-10-15T10:36:14.471+0000

  • log

• Name: Compress

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

  • Duration: 0 min 0 sec

  • Start Time: 2020-10-15T10:48:13.253+0000

  • log