Lazy load metrics listener

[amymin00]

What does this PR do?

Remove loading of metrics listener whenever using the extension. Instead, lazy import metrics listener only when user intends to send a distribution metric. Decreases loading of the extension by 57ms.

Motivation

Reducing layer sizes during serverless week

Testing Guidelines

Ran existing unit tests and integration tests. Ensured metrics are still sent in snapshots when sendDistributionMetric is called.

Uploaded layer version 150 with lazy loading changes and linked two serverless apps each with the layer. One app used the extension to transfer custom metrics, the second used a forwarder. Ran functions from each app several times and checked that metrics popped up appropriately in Datadog.

Custom metrics received through the extension:

Custom metrics received through a forwarder:

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

Merging #320 (e4d7bd2) into main (35295b3) will increase coverage by 0.19%.
The diff coverage is 96.00%.

❗ Current head e4d7bd2 differs from pull request most recent head 7c26b9a. Consider uploading reports for the commit 7c26b9a to get more accurate results

@@            Coverage Diff             @@
##             main     #320      +/-   ##
==========================================
+ Coverage   80.35%   80.55%   +0.19%     
==========================================
  Files          36       37       +1     
  Lines        1761     1779      +18     
  Branches      410      411       +1     
==========================================
+ Hits         1415     1433      +18     
+ Misses        294      293       -1     
- Partials       52       53       +1     

Impacted Files	Coverage Δ
src/index.ts	87.96% <94.73%> (+1.41%)	⬆️
src/metrics/extension.ts	100.00% <100.00%> (ø)
src/utils/extension-path.ts	100.00% <100.00%> (ø)

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[astuyve]

LGTM! We can release once we test against our self monitoring apps and confirm the behavior, but the integration tests passing here are already a strong indicator. Nice work!