[counters] Introduce two new counters categories, runtime and system.

The runtime category is for high level counters on runtime operations.
Things like number of threads, threadpool status, total memory consumption
and so on.

The system category is for synthetic counters that report system values
such as RSS, user time and load.
[counters] Introduce variance and units.
[counter] Kill counter type MONO_COUNTER_TIME_INTERVAL in favor of using a proper unit type.

This makes it more explicit and will further simplify the agent.
[counters] Add mono_counters_foreach API.
[counters] Simplify things a bit by removing unsigned counter types.