fix: pytest deadlock when using gevent

[brettlangdon]

Fixes #8281

This fix resolves an issue when using pytest + gevent where the telemetry writer was eager initialized by pytest entrypoints loading of our plugin causing a potential dead lock.

Reproduction

The specific case that we targeted solving with this PR:

# test.py
import ddtrace

import gevent.monkey

gevent.monkey.patch_all()


def test_thing():
    pass

$ pytest test.py
$ pytest -p ddtrace -p ddtrace.pytest_bdd -p ddtrace.pytest_benchmark test.py
$ pytest -p no:ddtrace -p no:ddtrace.pytest_bdd -p no:ddtrace.pytest_benchmark test.py

Previously would dead-lock, but now will execute without problem.

Trade offs

We no longer get telemetry enabled and sending data just from import ddtrace.

Moving telemetry writer starting to ddtrace.bootstrap.preload will mean that any unhandled exceptions that occur before the telemetry writer can be started will not get reported.

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: brettlangdon/move.telemetry.init
Commit report: 1f8f670
Test service: dd-trace-py

✅ 0 Failed, 173833 Passed, 1133 Skipped, 11h 29m 47.21s Total duration (20m 39.68s time saved)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 6.65%. Comparing base (3ea8bd8) to head (cdf3420).

Files	Patch %	Lines
tests/ci_visibility/test_cli.py	0.00%	10 Missing ⚠️
ddtrace/internal/runtime/runtime_metrics.py	0.00%	8 Missing ⚠️
tests/telemetry/test_telemetry.py	0.00%	6 Missing ⚠️
ddtrace/bootstrap/preload.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9141       +/-   ##
===========================================
- Coverage   78.55%    6.65%   -71.90%     
===========================================
  Files        1273     1244       -29     
  Lines      120300   118535     -1765     
===========================================
- Hits        94496     7886    -86610     
- Misses      25804   110649    +84845     

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[tonyredondo]

LGTM

[github-actions]

The backport to 2.8 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.8 2.8
# Navigate to the new working tree
cd .worktrees/backport-2.8
# Create a new branch
git switch --create backport-9141-to-2.8
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9e862f9617351fc88cd6e7c1150100495cf077e9
# Push it to GitHub
git push --set-upstream origin backport-9141-to-2.8
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.8

Then, create a pull request where the base branch is 2.8 and the compare/head branch is backport-9141-to-2.8.