Change sourcemap logging

[simitt]

fixes #4119

Motivation/summary

Remove unnecessary log line for source mapping and change potentially interesting log line from warning to debug level.

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

Start APM Server with source mapping enabled

• send request to RUM endpoint without service.name, check that nothing is logged.
• send request to RUM endpoint for which no sourcemap exists, or ensure the connection to ES is failing, check that nothing is logged.

Change log level to debug and repeat sending a request for which no sourcemap can be retrieved - a message should be logged

Related issues

#4119

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #4132 updated]

• Start Time: 2020-09-03T05:57:48.301+0000

• Duration: 50 min 4 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3255
Skipped	142
Total	3397

Steps errors

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• Name: Compress

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

  • Duration: 0 min 0 sec

  • Start Time: 2020-09-03T06:14:34.815+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: 2020-09-03T06:28:25.194+0000

  • log

• Name: Test Sync

  • Description: ./script/jenkins/sync.sh

  • Duration: 3 min 59 sec

  • Start Time: 2020-09-03T06:09:13.354+0000

  • log

[jalvz]

@simitt Im confused by the test instructions, service.name is required in the payload, isnt it?

[simitt]

There was a bug in the intake validation, so that service.name was allowed to be empty (see #4141); since it is fixed now, the test instructions are outdated; I suggest you test with empty service.version.