fix: guard against potential pkg_resources race (#14456)

We handle a potential race condition with the import of pkg_resources
that can happen when a module watchdog is enabled while the import of
said module is in progress.

Addresses #14449.

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(cherry picked from commit c0bcf90)

Author: P403n1x87

ddtrace/internal/module.py

@@ -320,7 +320,23 @@ def __init__(self) -> None:
         # NotImplementedError: Can't perform this operation for unregistered
         pkg_resources = sys.modules.get("pkg_resources")
         if pkg_resources is not None:
-            pkg_resources.register_loader_type(_ImportHookChainedLoader, pkg_resources.DefaultProvider)
+            for _ in range(5):
+                # The package might be in the process of being imported by
+                # another thread by the time this module watchdog is created. If
+                # we fail to find the method we try again after a short wait.
+                # This is likely to happen when a module watchdog is installed
+                # lazily after boot, e.g. as a response to a RC enablement
+                # record. This means that we are unlikely to add delays to the
+                # main or any other use threads.
+                try:
+                    pkg_resources.register_loader_type(_ImportHookChainedLoader, pkg_resources.DefaultProvider)
+                    break
+                except AttributeError:
+                    from time import sleep
+
+                    sleep(0.1)
+            else:
+                log.warning("Cannot ensure correct support with pkg_resources")
 
     def _add_to_meta_path(self) -> None:
         sys.meta_path.insert(0, self)  # type: ignore[arg-type]

releasenotes/notes/fix-pkg_resources-race-9e50201791f4cb98.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed an issue that could have caused some products to fail to start
+    properly in applications that use ``pkg_resources``, either directly or
+    indirectly.