Decouple Exporter creation with the Reentrant processor

[lalitb]

Changes

Nit fix, to enable exporter creation separately to the Reenterant processor, to make it consistent with the approach used with SimpleLogProcessor and BatchLogProcessor. No changes in functionality as such, though it is a breaking change for customers.

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed
• [ ] Unit tests added/updated (if applicable)
• [x] Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• [x] Changes in public API reviewed (if applicable)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 55.7%. Comparing base (56a6f45) to head (e67876a). Report is 14 commits behind head on main.

Files	Patch %	Lines
...pentelemetry-user-events-logs/src/logs/exporter.rs	0.0%	1 Missing :warning:
...ser-events-logs/src/logs/reentrant_logprocessor.rs	0.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #82 +/- ## ======================================= + Coverage 52.3% 55.7% +3.4% ======================================= Files 38 38 Lines 4967 5394 +427 ======================================= + Hits 2598 3006 +408 - Misses 2369 2388 +19 ```

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[lalitb]

Please callout the changes in changelog too.

Done.