Support metrics for web UI natively instead of using prometheus

[mwylde]

The web UI has a nice feature that shows commonly-used job metrics directly in the pipeline detail page, including event rate metrics and backpressure metrics. Today, this works like this:

  [Arroyo worker] --pushes--> [Prometheus push gateway] <--reads-- [Prometheus]
                                                                     ↑ queries
                                                                   [Arroyo API]

requiring prometheus and pushgateway in order for the web ui metrics to work.

As part of the effort in 0.11 to remove all external dependencies (following on #626), this PR removes the need for prometheus by adding a simple metrics system within the controller.

Now, the controller queries metrics from the workers, computes rates using a exponentially-weighted moving average, stores a limited history (5 minutes), and responds to queries from the API for those metrics.