Update monitoring agent report format

This commit expands the monitoring agent design document improving the
description of the general report file format.

There can be two kinds of data collectors (performance and status collectors).

Every collector can now be part of a category of collectors (such as
storage, instance, daemon, hypervisor, etc.) that defines a minimum basic
structure that has to be common to all the collectors part of the same class.

Also, such basic structure is defined for all the currently envisioned class
of collectors.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>