fix(pubsublite): set a default grpc connection pool size of 8 (#4462)

To ensure most users don't hit the limit of 100 streams per connection, if they have a high number of topic partitions.

pubsublite/internal/wire/rpc.go

@@ -192,24 +192,30 @@ func defaultClientOptions(region string) []option.ClientOption {
 	}
 }
 
+func streamClientOptions(region string) []option.ClientOption {
+	// To ensure most users don't hit the limit of 100 streams per connection, if
+	// they have a high number of topic partitions.
+	return append(defaultClientOptions(region), option.WithGRPCConnectionPool(8))
+}
+
 // NewAdminClient creates a new gapic AdminClient for a region.
 func NewAdminClient(ctx context.Context, region string, opts ...option.ClientOption) (*vkit.AdminClient, error) {
 	options := append(defaultClientOptions(region), opts...)
 	return vkit.NewAdminClient(ctx, options...)
 }
 
 func newPublisherClient(ctx context.Context, region string, opts ...option.ClientOption) (*vkit.PublisherClient, error) {
-	options := append(defaultClientOptions(region), opts...)
+	options := append(streamClientOptions(region), opts...)
 	return vkit.NewPublisherClient(ctx, options...)
 }
 
 func newSubscriberClient(ctx context.Context, region string, opts ...option.ClientOption) (*vkit.SubscriberClient, error) {
-	options := append(defaultClientOptions(region), opts...)
+	options := append(streamClientOptions(region), opts...)
 	return vkit.NewSubscriberClient(ctx, options...)
 }
 
 func newCursorClient(ctx context.Context, region string, opts ...option.ClientOption) (*vkit.CursorClient, error) {
-	options := append(defaultClientOptions(region), opts...)
+	options := append(streamClientOptions(region), opts...)
 	return vkit.NewCursorClient(ctx, options...)
 }