Cherry-pick of PR #19552 to 6.8 branch. Original message:

What does this PR do?

Handles OOM on long lines gracefully, writes the error into log.

• Addresses Filebeat OOMs on very long lines #19500

Why is it important?

Handle bad data gracefully without OOM.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• I just recently started looking at the beats code so might not have enough knowledge of any possible side-effects of this change. Please review with that in mind.

How to test this PR locally

Refer to #19500 for test case.
The filebeat doesn't get killed due to OOM with this change and the error is logged (see the screenshot attached).

Related issues

• Addresses Filebeat OOMs on very long lines #19500

Screenshots

Logs

2020-07-01T13:15:17.630Z	INFO	[crawler]	beater/crawler.go:108	Loading and starting Inputs completed. Enabled inputs: 1
2020-07-01T13:15:17.633Z	INFO	log/harvester.go:297	Harvester started for file: /test/readme.log
2020-07-01T13:15:17.635Z	ERROR	log/harvester.go:330	Read line error: exceeded max bytes in line limit; File: /test/readme.log