Panic count metrics could possibly be combined for easier analysis

Propolis maintains two metrics counting panics in the guest, and distinguishes between guest- and host-handled panics as different timeseries. Here:

propolis/bin/propolis-server/src/lib/stats/pvpanic.rs

Line 30 in 4ccbb04

struct PvPanicGuestHandled {

and here:

propolis/bin/propolis-server/src/lib/stats/pvpanic.rs

Line 39 in 4ccbb04

struct PvPanicHostHandled {

We may want to collapse these into one timeseries counting total panics, with a field to indicate who handled it (guest or host). But it's not clear from the PVPANIC documentation whether these two are mutually exclusive. If not, two timeseries would be needed. But if so, we may want to collapse them.