Retry when contacting the metadata server in platform detection
Retry when contacting the metadata server in platform detection

This matches the behavior of ComputeCredential from
Google.Apis.Auth, but the code to contact the Kubernetes service
host hasn't been changed yet. To separate them properly, the
previously-common HTTP timeout has been split.

googleapis/gax-dotnet,jskeet/gax-dotnet,googleapis/gax-dotnet,jskeet/gax-dotnet