fix(pylibmc): support client initialization using servers [backport 3.2] (#12785)

Backport e08d99c from #12721 to 3.2.

Currently calling `Client(servers=[url])` throws an error as seen with
the new test:

[Example CI
error](https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/847335230)
```
FAILED tests/contrib/pylibmc/test.py::TestPylibmcPatch::test_client_with_servers_option - TypeError: __init__() got multiple values for argument 'servers'
FAILED tests/contrib/pylibmc/test.py::TestPylibmcPatch::test_client_with_servers_option - TypeError: __init__() got multiple values for argument 'servers'
FAILED tests/contrib/pylibmc/test.py::TestPylibmcPatch::test_client_with_servers_option - TypeError: __init__() got multiple values for argument 'servers'
``` 

This is a bug in our integration logic because you can declare Clients
with and without the `servers` portion:
https://sendapatch.se/projects/pylibmc/reference.html .

**Why did this bug occur?**

When you call `pylibmc.Client(servers=[url])`, by the time it hits our
patch code, `**kwargs` already stores `servers`. Then when this gets
passed into the Client init, it sees two "servers"
https://github.com/lericson/pylibmc/blob/1183e48eb86adf0c23c5164bfe2c2a4058234640/src/pylibmc/client.py#L126,
hence the complaint about the multiple server arguments.

A quick fix is to check if the kwargs contains servers and set up the
client without duplicating it.



Fixes APMS-15146

## 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: wantsui <[email protected]>
Co-authored-by: erikayasuda <[email protected]>

Author: 3 people

ddtrace/contrib/internal/pylibmc/client.py

@@ -41,7 +41,7 @@ def __init__(self, client=None, service=memcached.SERVICE, tracer=None, *args, *
         if not isinstance(client, _Client):
             # We are in the patched situation, just pass down all arguments to the pylibmc.Client
             # Note that, in that case, client isn't a real client (just the first argument)
-            client = _Client(client, *args, **kwargs)
+            client = _Client(kwargs.get("servers", client), **{k: v for k, v in kwargs.items() if k != "servers"})
         else:
             log.warning(
                 "TracedClient instantiation is deprecated and will be remove "

releasenotes/notes/pylibmc-bug-fix-client-419c7912f54418b7.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    pylibmc: fixes an issue where using ``Client(server=[url])`` would throw the error ``__init__() got multiple values for argument 'servers'``

tests/contrib/pylibmc/test.py

@@ -371,6 +371,36 @@ def test_patch_unpatch(self):
         assert spans, spans
         assert len(spans) == 1
 
+    def test_client_with_servers_option(self):
+        """
+        A test to make sure we support using `servers`, ie: with pylibmc.Client(servers=[url])
+        """
+        url = "%s:%s" % (cfg["host"], cfg["port"])
+
+        patch()
+        client = pylibmc.Client(servers=[url])
+        assert client.addresses[0] is url
+        Pin.get_from(client)._clone(service=self.TEST_SERVICE, tracer=self.tracer).onto(client)
+        client.set("a", 1)
+        spans = self.pop_spans()
+        assert spans, spans
+        assert len(spans) == 1
+
+    def test_client_without_servers_option(self):
+        """
+        A test to make sure we support the most basic use case of calling pylibmc.Client([url])
+        """
+        url = "%s:%s" % (cfg["host"], cfg["port"])
+
+        patch()
+        client = pylibmc.Client([url])
+        assert client.addresses[0] is url
+        Pin.get_from(client)._clone(service=self.TEST_SERVICE, tracer=self.tracer).onto(client)
+        client.set("a", 1)
+        spans = self.pop_spans()
+        assert spans, spans
+        assert len(spans) == 1
+
 
 class TestPylibmcPatchSchematization(TestPylibmcPatchDefault):
     @TracerTestCase.run_in_subprocess(env_overrides=dict(DD_SERVICE="mysvc", DD_TRACE_SPAN_ATTRIBUTE_SCHEMA="v0"))