[Elastic Agent] Fix logging read from runnings process

[ph]

Since running process are using the debug level by default now, we have
a log of messages going through the log.

This commit add multiline support reading.

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• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

💚 Build Succeeded

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• Build Cause: [Pull request [Elastic Agent] Fix logging read from runnings process #18275 updated]

• Start Time: 2020-05-06T08:21:31.371+0000

• Duration: 32 min 24 sec (1944347)

• Commit: 72ce7b1

[michalpristas]

LGTM, change tested locally with suggested change,

[ruflin]

Opened a backport here to get this green in time: #18285

[ph]

thanks both of you