uninitialized constant Gapic::ServiceStub::ChannelPool

Describe the bug:
Getting uninitialized constant Gapic::ServiceStub::ChannelPool when running the get_account_hierarchy example

https://github.com/googleads/google-ads-ruby/blob/51e655cebb578cb5a9bf60ccc6732e2ac652c9ba/examples/account_management/get_account_hierarchy.rb#L45
Steps to Reproduce:

1. clone this repo locally
2. configure ~/google_ads_config.rb with the required secrets
3. run ruby get_account_hierarchy.rb

Expected behavior:
Did not expect uninitialized constant Gapic::ServiceStub::ChannelPool

Client library version and API version:
Client library version: 25.0.1
Google Ads API version: V15

Request/Response Logs:
➜ script git:(develop) ✗ ruby get_account_hierarchy.rb

No manager customer ID is specified. The example will print the hierarchies of all accessible customer IDs:
/Users/xxx/.rbenv/versions/3.1.0/lib/ruby/gems/3.0.0/gems/google-ads-googleads-25.0.1/lib/google/ads/google_ads/v15/services/customer_service/client.rb:570:in `channel_pool': uninitialized constant Gapic::ServiceStub::ChannelPool (NameError)
        from /Users/xxx/.rbenv/versions/3.1.0/lib/ruby/gems/3.0.0/gems/google-ads-googleads-25.0.1/lib/google/ads/google_ads/v15/services/customer_service/client.rb:141:in `initialize'
        from /Users/xxx/.rbenv/versions/3.1.0/lib/ruby/gems/3.0.0/gems/google-ads-googleads-25.0.1/lib/google/ads/google_ads/factories/v15/services.rb:1420:in `new'
        from /Users/xxx/.rbenv/versions/3.1.0/lib/ruby/gems/3.0.0/gems/google-ads-googleads-25.0.1/lib/google/ads/google_ads/factories/v15/services.rb:1420:in `customer'
        from /Users/xxx/.rbenv/versions/3.1.0/lib/ruby/3.0.0/delegate.rb:87:in `method_missing'
        from get_account_hierarchy.rb:145:in `get_account_hierarchy'
        from get_account_hierarchy.rb:185:in `<main>'

Anything else we should know about your project / environment:

Reproduce the failure with examples/account_management/get_account_hierarchy.rb using the documented configuration. Start at the channel_pool entry point in the generated customer_service/client.rb around line 570 and inspect why Gapic::ServiceStub::ChannelPool is unavailable. Done means the example initializes CustomerService without the NameError.