Add gc metric for allocation per second

[alfh]

For now, I just want feedback if this is something you also see as useful.
If you see it as useful, I will also add promotion per second.

The reason for having the alloction / promotion per second, is that when just using the "promotion" and "allocation" since last garbage collect, I get a constant number, for example 100MB of allocation, because the garbage collect kicks in after a number of bytes has been allocated. So sometimes there is 10 seconds between the garbage collect and sometimes there is 2 hours.

So I need the rate per second, to be able to see how "much" allocation is actually happening "per second".