Add support for Google Pub/Sub as transport broker (#2147)

* Add support for Google Pub/Sub as transport broker

 * Add tests
 * Add docs

* flake8

* flake8

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* Fix future import

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add missing test requirements

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add monitoring dependency

* Fix test for python3.8

* Mock better google's monitoring api

* Flake8

* Add refdoc

* Add extra url to workaround pypy grpcio

* Add extra index url in tox for grpcio/pypy support

* Revert "Add extra url to workaround pypy grpcio"

This reverts commit dfde4d5.

* pin grpcio version to match extra_index wheel

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* Reduce poll calls if qos denies msg rx

---------

Co-authored-by: Haim Daniel <haimdaniel@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tomer Nosrati <tomer.nosrati@gmail.com>

docs/reference/kombu.transport.gcpubsub.rst

@@ -0,0 +1,24 @@
+==============================================================
+ Google Cloud Pub/Sub Transport - ``kombu.transport.gcpubsub``
+==============================================================
+
+.. currentmodule:: kombu.transport.gcpubsub
+
+.. automodule:: kombu.transport.gcpubsub
+
+    .. contents::
+        :local:
+
+    Transport
+    ---------
+
+    .. autoclass:: Transport
+        :members:
+        :undoc-members:
+
+    Channel
+    -------
+
+    .. autoclass:: Channel
+        :members:
+        :undoc-members:

kombu/transport/__init__.py

@@ -43,7 +43,8 @@ def supports_librabbitmq() -> bool | None:
     'etcd': 'kombu.transport.etcd:Transport',
     'azurestoragequeues': 'kombu.transport.azurestoragequeues:Transport',
     'azureservicebus': 'kombu.transport.azureservicebus:Transport',
-    'pyro': 'kombu.transport.pyro:Transport'
+    'pyro': 'kombu.transport.pyro:Transport',
+    'gcpubsub': 'kombu.transport.gcpubsub:Transport',
 }
 
 _transport_cache = {}