  Metallicity is theoretically thought to be a fundamental driver in gamma-ray
burst (GRB) explosions and energetics, but is still, even after more than a
decade of extensive studies, not fully understood. This is largely related to
two phenomena: a dust-extinction bias, that prevented high-mass and thus likely
high-metallicity GRB hosts to be detected in the first place, and a lack of
efficient instrumentation, that limited spectroscopic studies including
metallicity measurements to the low-redshift end of the GRB host population.
The subject of this work is the very energetic GRB 110918A, for which we
measure a redshift of z=0.984. GRB 110918A gave rise to a luminous afterglow
with an intrinsic spectral slope of b=0.70, which probed a sight-line with
little extinction (A_V=0.16 mag) typical of the established distributions of
afterglow properties. Photometric and spectroscopic follow-up observations of
the galaxy hosting GRB 110918A, including optical/NIR photometry with GROND and
spectroscopy with VLT/X-shooter, however, reveal an all but average GRB host in
comparison to the z~1 galaxies selected through similar afterglows to date. It
has a large spatial extent with a half-light radius of ~10 kpc, the highest
stellar mass for z<1.9 (log(M_*/M_sol) = 10.68+-0.16), and an Halpha-based star
formation rate of 41 M_sol/yr. We measure a gas-phase extinction of ~1.8 mag
through the Balmer decrement and one of the largest host-integrated
metallicities ever of around solar (12 + log(O/H) = 8.93+/-0.13). This presents
one of the very few robust metallicity measurements of GRB hosts at z~1, and
establishes that GRB hosts at z~1 can also be very metal rich. It conclusively
rules out a metallicity cut-off in GRB host galaxies and argues against an
anti-correlation between metallicity and energy release in GRBs.
