Change extension detection mechanism

[DarcyRaynerDD]

What does this PR do?

Changes the extension detection mechanism to check if the extension binary is present locally. Also, moves calls to extension outside of root trace span.

Motivation

Currently, checking for the datadog extension is done by attempting to open a tcp connection to the extension's endpoints on localhost. When the extension is present this request will fail. This has the side effect of generating an error span in our trace on cold starts. Instead, we swap to checking for access to the extension file first, before making the tcp connection.

The second change we make is to call extension methods, (including flush), outside of the main trace. This reduces the noise in the trace, and means the trace can now be flushed on the same invocation that created it.

Testing Guidelines

Updated integration tests, and added unit tests.

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-io]

Codecov Report

Merging #131 (f7c3082) into master (3693796) will decrease coverage by 0.04%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
- Coverage   87.68%   87.64%   -0.05%     
==========================================
  Files          29       29              
  Lines         958      971      +13     
  Branches      169      169              
==========================================
+ Hits          840      851      +11     
- Misses         80       82       +2     
  Partials       38       38              

Impacted Files	Coverage Δ
src/index.ts	83.33% <88.23%> (-1.45%)	⬇️
src/metrics/extension.ts	100.00% <100.00%> (ø)

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[Czechh]

👏 - love the refactor