Harden sink / metric creation against transient errors. (#6180)
Harden sink / metric creation against transient errors. (#6180)

Closes #5931.

dhermes/gcloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,dhermes/google-cloud-python,dhermes/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,jonparrott/gcloud-python,GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,jonparrott/google-cloud-python,googleapis/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python,jonparrott/gcloud-python,GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python