[agent] agent metrics

As a user who has started an agent, in order to have confidence the agent was deployed successfully and is healthy, I would like to be able see the following agent metrics in the shell panels and SDK: All are "instantaneous" values taken as averages over the last 1min: - [ ] disk utilization GB - [ ] network upload speed in Kbps - [ ] network download speed in Kbps - [ ] agent `startedAt` date This information is not intended for in-depth diagnosis of problems, so it need not be too precise. See also: - #6310