Do not call flush extension after each invocation.

[purple4reina]

What does this PR do?

Only calls Flush() on the extension when running in local test mode.

See DataDog/datadog-lambda-python#406

Motivation

This cuts runtime duration of a hello world function by about 10%.

Testing Guidelines

Additional Notes

Types of changes

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

Checklist

• 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

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (73ccca7) 81.90% compared to head (db96efc) 82.07%.

❗ Current head db96efc differs from pull request most recent head 156f596. Consider uploading reports for the commit 156f596 to get more accurate results

Files	Patch %	Lines
internal/metrics/listener.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   81.90%   82.07%   +0.17%     
==========================================
  Files          13       13              
  Lines         884      887       +3     
==========================================
+ Hits          724      728       +4     
+ Misses        131      129       -2     
- Partials       29       30       +1     

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[duncanista]

LGTM – thanks for adding the 1 or true check.