This is just the beginning -- Silverline also sneaks its way into the scheduling algorithms controlling the server. Once you put an application in a Silverline "container," Silverline can limit the resources it can consume, a process Librato calls "workload virtualization." Background tasks such as disk defragmentation can be given minimal headroom. High-computation tasks like converting photos or videos can also be limited so that a burst of uploads won't swamp your machines. You can control these spikes more easily at the OS level.
Librato Silverline monitors the essential metrics of the system. This graph shows the rise in network write.
This is, I think, a great idea. For a number of years, people have punted on all of the trouble with the OS configuration, instead setting up another virtual server running another copy of the OS. Every application was pushed off into its own virtual server even if this isolation wasn't required. This is largely a waste of OS copies, and it creates as many problems as it solves. Perhaps we can go back to one layer of the OS running directly on the iron without any virtualization sucking up CPU cycles.
The pricing of the Metrics service is set up like a taxi meter: You pay for every data item you store. Sending 50 measurements every minute costs $4.46 per month, for example. Librato provides an HTML5 calculator so that you can come up with your own measurements, but the rates seem to be entirely linear. Silverline is priced at $0.006 per CPU core hour. Librato's online calculator indicates that a month of virtual control over a single-core server would cost $4.38.