  Metallic spin liquid has been reported in several correlated metals, but a
satisfactory theoretical description is not yet available. Here we propose a
potential route to realize the metallic spin liquid and construct an effective
$\mathbb{Z}_2$ gauge theory with charged fractionalized excitations on the
triangular Kondo lattice. This leads to a $\mathbb{Z}_2$ metallic spin liquid
featured with long-lived, heavy holon excitations of spin 0 and charge $+e$ and
a partially enlarged electron Fermi surface. It differs from the weak-coupling
FL$^*$ state proposed earlier and may be viewed as a fractionalized heavy
fermion liquid. Our theory provides a general framework to describe the
metallic spin liquid in frustrated Kondo lattice systems.
