Add instructions for manual tracer initialization

[tianchu]

What does this PR do?

Update the docs with instructions for manual tracer initialization.

Motivation

Need to manually initialize the tracer to apply custom settings.

Testing Guidelines

Instructions have been manually tested.

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 #122 (92a1cb4) into main (4676b80) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   86.19%   86.19%           
=======================================
  Files          31       31           
  Lines        1181     1181           
  Branches      235      235           
=======================================
  Hits         1018     1018           
  Misses        108      108           
  Partials       55       55           

---

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.19%. Comparing base (4676b80) to head (92a1cb4).
⚠️ Report is 383 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   86.19%   86.19%           
=======================================
  Files          31       31           
  Lines        1181     1181           
  Branches      235      235           
=======================================
  Hits         1018     1018           
  Misses        108      108           
  Partials       55       55           

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