[torreypayne]

Fixes two deterministic failures in google-cloud-monitoring/samples:

• Pass page_size: 100 to list_metric_descriptors in metrics.rb so single-page responses stay below gRPC's 4 MiB default limit (4212350 vs. 4194304 on helical-zone-771).
• Fix retry_block in acceptance/helper.rb to retry count attempts with capped exponential backoff (<= 15s), and use retry_block 12 for custom metric visibility in metrics_test.rb and quickstart_test.rb.

Verified rubocop, syntax checks, and request proto compatibility across Ruby 3.2, 3.3, and 4.0.

Related to b/552162906, b/580634043, and b/580635694.