x/playground: over quota for compute.googleapis.com ListInstances

[toothrot]

The playground project is going over quota for ListInstances, likely from the poller in gcpdial: https://github.com/golang/playground/blob/15e968511d3060ae3d38f53ab43cc48529d26825/internal/gcpdial/gcpdial.go#L190

We should introduce a ticker here to not hit our quota of 10000 requests per 100 seconds.

[gopherbot]

Change https://golang.org/cl/227648 mentions this issue: playground/internal/gcpdial: throttle polling for instances

[toothrot]

This has been resolved. Down from 80QPS to ~40.