  We investigate the metal species associated with the Ly$\alpha$ forest in the
eBOSS quasar spectra. Metal absorption lines are revealed in the stacked
spectra from cross-correlating the selected Ly$\alpha$ absorbers in the forest
and the flux fluctuation field. Up to 13 metal species are identified
associated with relatively strong Ly$\alpha$ absorbers (those with flux
fluctuation $-1.0<\delta_{\rm Ly\alpha}<-0.6$ and with neutral hydrogen column
density of ~ $10^{15-16}$ $\rm cm^{-2}$) over absorber redshift range of
$2<z_{\rm abs}<4$. The column densities of these species decrease toward higher
redshift and for weaker Ly$\alpha$ absorbers. From modelling the column
densities of various species, we find that the column density pattern suggests
contributions from multiple gas components both in the circumgalactic medium
(CGM) and in the intergalactic medium (IGM). While the low-ionization species
(e.g., C II, Si II, and Mg II) can be explained by high-density, cool gas ($T
\sim 10^4$ K) from the CGM, the high-ionization species may reside in
low-density or high-temperature gas in the IGM. The measurements provide inputs
to model metal contamination in the Ly$\alpha$ forest baryon acoustic
oscillations measurement. Comparison with metal absorptions in high-resolution
quasar spectra and with hydrodynamic galaxy formation simulations can further
elucidate the physical conditions of these Ly$\alpha$ absorbers.
