feat: Skip the flush() call unless the DD_LOCAL_TEST env var is set

[astuyve]

What does this PR do?

Removes the call to /flush, as the telemetry-api tells us this information now

Fixes #452

Motivation

We'd like to remove this entirely but it's required for people using the extension in a local testing mode. We don't intend to further support local testing in this library at this time.

Testing Guidelines

Additional Notes

Types of Changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (761b3dc) 82.06% compared to head (7440d6e) 82.09%.

:exclamation: Current head 7440d6e differs from pull request most recent head cde829b. Consider uploading reports for the commit cde829b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #454 +/- ## ========================================== + Coverage 82.06% 82.09% +0.03% ========================================== Files 54 54 Lines 2180 2184 +4 Branches 507 509 +2 ========================================== + Hits 1789 1793 +4 Misses 329 329 Partials 62 62 ```

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