feat: support sending metrics before handler initialization

[duncanista]

What does this PR do?

Allows customer to use the API sendDistributionMetric and sendDistributionMetricWithDate outside of the handler.

Motivation

#515

Testing Guidelines

• Added unit tests
• Added integration test
• Tested manually

Additional Notes

SVLS-4633

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

Attention: Patch coverage is 81.39535% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 81.89%. Comparing base (dadad52) to head (5b2885d).

❗ Current head 5b2885d differs from pull request most recent head 59d713d. Consider uploading reports for the commit 59d713d to get more accurate results

Files	Patch %	Lines
src/index.ts	60.00%	6 Missing ⚠️
src/utils/log.ts	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #524      +/-   ##
==========================================
+ Coverage   81.75%   81.89%   +0.13%     
==========================================
  Files          54       55       +1     
  Lines        2209     2248      +39     
  Branches      515      519       +4     
==========================================
+ Hits         1806     1841      +35     
- Misses        337      343       +6     
+ Partials       66       64       -2     

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[joeyzhao2018]

LGTM