Metrics server should return a metrics rate value

[futurechimp]

At the moment, we've hardcoded the duration for metrics reporting. This causes trouble for us when node numbers increase, because the metrics server gets hammered if we expand the network significantly. Although metrics are not part of our security model (they're self-reported), they're still pretty useful for people to see whether their nodes are up and running.

Let's define the metrics reporting_period dynamically, where reporting_period = num_mixnodes / desired_traffic_rate. If we say we want to have e.g. a desired_traffic_rate of 50 requests per second hit the metrics API, and we have right now 600 mixnodes, then our reporting_period ends up being 30 seconds. Every time a mixnode submits a metrics report, it gets back the reporting_period of 30. When the network grows or shrinks, nodes will automatically adjust to send their traffic at lower or higher rates depending on what the metrics server can support.