Collect (opt-in) usage metrics

We're provided an endpoint that we can send metrics to.

The user needs to opt-in to it, the workflow should be:
- after install, we start collecting data on the client side only
- after a while (time-based? usage-based?) we ask the user whether they'd like to contribute, informing what kind of data would be collected and how (no identifying properties)
- we allow the user to see the most recent report we would send
- if the user agrees, we push the data to the collection service and do this periodically from then on
- otherwise, we send a single message only containing the information that the user did not opt-in

Ideas for data (some will be plain data, others averages or total counts, maybe we can have max/min for some as well?):
- OS
- OS version
- CPU(s)
- storage space
- storage type
- memory size
- number of instances
- aliases used to launch instances
- instances per OS (from lsb_release?)
- instances per OS version (ditto)
- disk size
- CPU count
- memory size
- instance uptime
- instance lifetime
- mounts used
- …