fix(lib-injection): don't close stdout/stderr (#13926)

# Overview
This commit resolves an issue where telemetry forwarding never completes
when DD_APM_INSTRUMENTATION_DEBUG is enabled. We are closing the pipe
from the Python side, which means the telemetry forwarder will fill the
buffer and be unable to complete.

# Test
I updated the `telemetry-forwarder.sh` to look like the following:
```bash
#!/usr/bin/env bash

# Generate 64KB of output on stderr (writes to fd 2)
# This creates 1024 lines of 64 bytes each = 64KB
for i in {1..1024}; do
  echo "This is a line of telemetry data to fill the pipe buffer ---- $i" >&2
done

# Write stdin + $1 to a file as before
echo "$1	$(</dev/stdin)" >>mock-telemetry.out
```

I then ran this on main and this branch to confirm `mock-telemetry.out`
is written on this branch but never gets written on main:
```bash
DD_APM_INSTRUMENTATION_DEBUG="true" DD_INJECTION_ENABLED=true DD_TRACE_DEBUG=true PYTHONPATH=/Users/mark.spicer/go/src/github.com/DataDog/dd-trace-py/lib-injection/sources:$PYTHONPATH DD_TELEMETRY_FORWARDER_PATH=/Users/mark.spicer/go/src/github.com/DataDog/dd-trace-py/lib-injection/sources/telemetry-forwarder.sh python3 /Users/mark.spicer/go/src/github.com/DataDog/injector-dev/apps/python/app.py
```

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, 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](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Brett Langdon <brett.langdon@datadoghq.com>

Author: betterengineering

lib-injection/sources/sitecustomize.py

@@ -182,8 +182,8 @@ def _send_telemetry(event):
     p = subprocess.Popen(
         [FORWARDER_EXECUTABLE, "library_entrypoint"],
         stdin=subprocess.PIPE,
-        stdout=subprocess.PIPE,
-        stderr=subprocess.PIPE,
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
         universal_newlines=True,
     )
     # Mimic Popen.__exit__ which was added in Python 3.3
@@ -199,10 +199,6 @@ def _send_telemetry(event):
     finally:
         if p.stdin:
             p.stdin.close()
-        if p.stderr:
-            p.stderr.close()
-        if p.stdout:
-            p.stdout.close()
 
         # backwards compatible `p.wait(1)`
         start = time.time()

releasenotes/notes/resolve-telemetry-forwarder-bug-e71f154d954eda4c.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    lib-injection: Fix a bug preventing the Single Step Instrumentation (SSI) telemetry forwarder from completing when debug logging was enabled.