chore(telemetry): avoid starting the telemetry after a process forks (backport #3984) (#3995)

This is an automatic backport of pull request #3984 done by [Mergify](https://mergify.com).


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Author: mergify[bot]

ddtrace/internal/telemetry/writer.py

@@ -241,7 +241,6 @@ def _fork_writer(self):
         # Avoid sending duplicate events.
         # Queued events should be sent in the main process.
         self._reset_queues()
-        self.start()
 
     def disable(self):
         # type: () -> None

tests/tracer/telemetry/test_telemetry.py

@@ -63,3 +63,21 @@ def test_fork():
         assert telemetry.telemetry_writer.status == ServiceStatus.RUNNING
         # Kill the process so it doesn't continue running the rest of the test suite
         os._exit(0)
+
+
+def test_logs_after_fork(ddtrace_run_python_code_in_subprocess):
+    # Regression test: telemetry writer should not log an error when a process forks
+    _, err, status, _ = ddtrace_run_python_code_in_subprocess(
+        """
+import ddtrace
+import logging
+import os
+
+logging.basicConfig() # required for python 2.7
+ddtrace.internal.telemetry.telemetry_writer.enable()
+os.fork()
+""",
+    )
+
+    assert status == 0, err
+    assert err == b""