Don't send all the metrics on GET /api/metrics

[hariharan-uno]

As the number of metrics increases, it is unfeasible to respond with all the metrics as we are doing now.

https://github.com/codeignition/marksman/blob/master/metrics_handler.go#L19

Change it to respond with only the metrics from the last 5 or 10 seconds. May be add a query string to override the default.