[EXPORTER]: Elasticsearch exporter put log resource in root instead of under 'resources'

[ShadowMaxLeb]

In order to give the user full flexibility on what and where to put resources for logs, remove the fact that it was forced to always be under the resource.
This allows to have resources such as client.id that will be under the root and compliant with ECS format.

Closes #3119

Changes

Log resources are set under the root object and not under resources anymore.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.85%. Comparing base (497eaf4) to head (1a0d85e).
Report is 159 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3131      +/-   ##
==========================================
+ Coverage   87.12%   87.85%   +0.73%     
==========================================
  Files         200      195       -5     
  Lines        6109     6136      +27     
==========================================
+ Hits         5322     5390      +68     
+ Misses        787      746      -41     

see 100 files with indirect coverage changes

[marcalff]

LGTM, thanks for the fix.

[marcalff]

@ShadowMaxLeb

In the CHANGELOG.md, please add an entry for important changes, to document that resources are no longer added under "root/resource" but are added under root instead.

I will merge this PR after that.

Thanks for the fix.

[ShadowMaxLeb]

@marcalff done

[marcalff]

@marcalff done

Thanks.

I meant a section similar to this:

Important changes:

* [API] Jaeger Propagator should not be deprecated
  [#3086](https://github.com/open-telemetry/opentelemetry-cpp/pull/3086)

  * Deprecation of the Jaeger propagator, as announced on 2023-01-31
    in version 1.8.2, is now reverted.
  * This deprecation turned out to be not justified,
    as the Jaeger propagator can be used without the (now removed)
    Jaeger exporter.

but since there is a note in the changelog, I will notice it and edit it when doing the next release, so this will do for now.

Merging.