fix: instance grpc client once per process in benchmarks. 

Other Changes
* One gRPC client per process
* One async io event loop per process
* One process pool per benchmark. 

Fixes b/474536663 , b/473669384

PR created by the Librarian CLI to initialize a release. Merging this PR
will auto trigger a release.

Librarian Version: v1.0.2-0.20251119154421-36c3e21ad3ac
Language Image:
us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:8e2c32496077054105bd06c54a59d6a6694287bc053588e24debe6da6920ad91
<details><summary>google-cloud-storage: 3.9.0</summary>

##
[3.9.0](googleapis/python-storage@v3.8.0...v3.9.0)
(2026-02-02)

### Features

* update generation for MRD (#1730)
([08bc7082](googleapis/python-storage@08bc7082))

* add get_object method for async grpc client (#1735)
([0e5ec29b](googleapis/python-storage@0e5ec29b))

* Add micro-benchmarks for reads comparing standard (regional) vs rapid
(zonal) buckets. (#1697)
([1917649f](googleapis/python-storage@1917649f))

* Add support for opening via `write_handle` and fix `write_handle` type
(#1715)
([2bc15fa5](googleapis/python-storage@2bc15fa5))

* add samples for appendable objects writes and reads
([2e1a1eb5](googleapis/python-storage@2e1a1eb5))

* add samples for appendable objects writes and reads (#1705)
([2e1a1eb5](googleapis/python-storage@2e1a1eb5))

* add context manager to mrd (#1724)
([5ac2808a](googleapis/python-storage@5ac2808a))

* Move Zonal Buckets features of `_experimental` (#1728)
([74c9ecc5](googleapis/python-storage@74c9ecc5))

* add default user agent for grpc (#1726)
([7b319469](googleapis/python-storage@7b319469))

* expose finalized_time in blob.py applicable for GET_OBJECT in ZB
(#1719)
([8e21a7f](googleapis/python-storage@8e21a7fe))

* expose `DELETE_OBJECT` in `AsyncGrpcClient` (#1718)
([c8dd7a0b](googleapis/python-storage@c8dd7a0b))

* send `user_agent` to grpc channel (#1712)
([cdb2486b](googleapis/python-storage@cdb2486b))

* integrate writes strategy and appendable object writer (#1695)
([dbd162b3](googleapis/python-storage@dbd162b3))

* Add micro-benchmarks for writes comparing standard (regional) vs rapid
(zonal) buckets. (#1707)
([dbe9d8b8](googleapis/python-storage@dbe9d8b8))

* add support for `generation=0` to avoid overwriting existing objects
and add `is_stream_open` support (#1709)
([ea0f5bf8](googleapis/python-storage@ea0f5bf8))

* add support for `generation=0` to prevent overwriting existing objects
([ea0f5bf8](googleapis/python-storage@ea0f5bf8))

* add `is_stream_open` property to AsyncAppendableObjectWriter for
stream status check
([ea0f5bf8](googleapis/python-storage@ea0f5bf8))

### Bug Fixes

* receive eof while closing reads stream (#1733)
([2ef63396](googleapis/python-storage@2ef63396))

* update write handle on every recv() (#1716)
([5d9fafe1](googleapis/python-storage@5d9fafe1))

* implement requests_done method to signal end of requests in async
streams. Gracefully close streams. (#1700)
([6c160794](googleapis/python-storage@6c160794))

* implement requests_done method to signal end of requests in async
streams. Gracefully close streams.
([6c160794](googleapis/python-storage@6c160794))

* instance grpc client once per process in benchmarks (#1725)
([721ea2dd](googleapis/python-storage@721ea2dd))

* Fix formatting in setup.py dependencies list (#1713)
([cc4831d7](googleapis/python-storage@cc4831d7))

* Change contructors of MRD and AAOW AsyncGrpcClient.grpc_client to
AsyncGrpcClient (#1727)
([e730bf50](googleapis/python-storage@e730bf50))

</details>