Downgrade prometheus-client (#208)
Downgrade prometheus-client (#208)

The new version is incompatible with fluent-plugin-prometheus, and it's causing runtime errors. Unfortunately our presubmit tests don't seem to catch the fact that it violates the requirements in the fluent-plugin-prometheus gemspec.
GoogleCloudPlatform/google-fluentd,GoogleCloudPlatform/google-fluentd,GoogleCloudPlatform/google-fluentd