added script to get the rate of incoming firehose messages
added script to get the rate of incoming firehose messages

Signed-off-by: Matt Cholick <094c01862037bc46d432680065307673fb455b42@pivotal.io>

cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools