fix(telemetry): register after fork hooks on initialization (backport #4319) (#4421)

This is an automatic backport of pull request #4319 done by [Mergify](https://mergify.com).


---


<details>
<summary>Mergify commands and options</summary>

<br />

More conditions and actions can be found in the [documentation](https://docs.mergify.com/).

You can also trigger Mergify actions by commenting on this pull request:

- `@Mergifyio refresh` will re-evaluate the rules
- `@Mergifyio rebase` will rebase this PR on its base branch
- `@Mergifyio update` will merge the base branch into this PR
- `@Mergifyio backport <destination>` will backport this PR on `<destination>` branch

Additionally, on Mergify [dashboard](https://dashboard.mergify.com/) you can:

- look at your merge queues
- generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com
</details>

Author: mergify[bot]

ddtrace/internal/telemetry/writer.py

@@ -54,6 +54,7 @@ def __init__(self, agent_url=None):
         self._integrations_queue = []  # type: List[Dict]
         self._lock = forksafe.Lock()  # type: forksafe.ResetObject
         self._forked = False  # type: bool
+        forksafe.register(self._fork_writer)
 
         self._headers = {
             "Content-type": "application/json",
@@ -278,7 +279,6 @@ def disable(self):
         if self.status == ServiceStatus.STOPPED:
             return
 
-        forksafe.unregister(self._fork_writer)
         atexit.unregister(self.stop)
 
         self.stop()
@@ -295,5 +295,4 @@ def enable(self):
         self._enabled = True
         self.start()
 
-        forksafe.register(self._fork_writer)
         atexit.register(self.stop)