[7.x] docs: OpenTelemetry Elastic exporter (#3845)

[bmorelli25]

Backports the following commits to 7.x:

• docs: OpenTelemetry Elastic exporter (docs: OpenTelemetry Elastic exporter #3845)

[codecov-commenter]

Codecov Report

Merging #3859 into 7.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              7.x    #3859   +/-   ##
=======================================
  Coverage   79.95%   79.95%           
=======================================
  Files         137      137           
  Lines        6182     6182           
=======================================
  Hits         4943     4943           
  Misses       1239     1239           

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #3859 opened]

• Start Time: 2020-06-08T18:26:30.995+0000

• Duration: 27 min 9 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3020
Skipped	142
Total	3162

Steps errors

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• Name: Test Sync

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

  • Duration: 3 min 55 sec

  • Start Time: 2020-06-08T18:37:35.662+0000

  • log