Rename call_count to call_rate
Rename call_count to call_rate

I've now understood statsd counters better .. they are used to measure a
rate rather than an overall count. This change follows:

alphagov/whitehall@2f571288533773af97815e2f88430dd05fec61a9

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet