  We present the KODIAQ-Z survey aimed to characterize the cool, photoionized
gas at 2.2<z<3.6 in 202 HI-selected absorbers with 14.6<log N(HI)<20, i.e., the
gaseous interface between galaxies and the intergalactic medium (IGM). We find
that the 14.6<log N(HI)<20 gas at 2.2<z<3.6 can be metal-rich gas
(-1.6<[X/H]<-0.2) as seen in damped Ly-alpha absorbers (DLAs); it can also be
very metal-poor ([X/H]<-2.4) or even pristine gas ([X/H]<-3.8) not observed in
DLAs, but commonly observed in the IGM. For 16<log N(HI)<20 absorbers, the
frequency of pristine absorbers is about 1%-10%, while for 14.6<log N(HI)<16
absorbers it is 10%-20%, similar to the diffuse IGM. Supersolar gas is
extremely rare (<1%) in this gas. The factor of several thousand spread from
the lowest to highest metallicities and large metallicity variations (a factor
of a few to >100) between absorbers separated by less than 500 km/s imply that
the metals are poorly mixed in 14.6<log N(HI)<20 gas. We show that these
photoionized absorbers contribute to about 10% of the cosmic baryons and 30% of
the cosmic metals at 2.2<z<3.6. We find the mean metallicity increases with
N(HI), consistent with what is found in z<1 gas. The metallicity of gas in this
column density regime has increased by a factor ~8 from 2.2<z<3.6 to z<1, but
the contribution of the 14.6<log N(HI)<19 absorbers to the total metal budget
of the universe at z<1 is half that at 2.2<z<3.6, indicating a substantial
shift in the reservoirs of metals between these two epochs. We compare the
KODIAQ-Z results to FOGGIE cosmological zoom simulations. The simulations show
an evolution of [X/H] with N(HI) similar to our observational results. Very
metal-poor absorbers with [X/H]<-2.4 at z~2-3 in these simulations are
excellent tracers of inflows, while higher metallicity absorbers are a mixture
of inflows and outflows.
