auth: log warning for excessive gRPC pool size

## Is your feature request related to a problem? Please describe.

Excessive pool size is a likely cause of high error rates in token requests. See b/429104987#comment38 for full analysis.

## Describe the solution you'd like

Log a warning if the user configures gRPC pool size (using [option.WithGRPCConnectionPool](https://pkg.go.dev/google.golang.org/api/option#WithGRPCConnectionPool)) greater than a reasonable threshold based on usage guidance such as in https://cloud.google.com/bigtable/docs/configure-connection-pools#example

## Describe alternatives you've considered

Add similar guidance to the docs for `option.WithGRPCConnectionPool`.