  We present ultra-deep Keck/MOSFIRE rest-optical spectra of two star-forming
galaxies at z=2.18 in the COSMOS field with bright emission lines, representing
more than 20~hours of total integration. The fidelity of these spectra enabled
the detection of more than 20 unique emission lines for each galaxy, including
the first detection of the auroral [O II]$\lambda\lambda$7322,7332 lines at
high redshift. We use these measurements to calculate the electron temperature
in the low-ionization O$^+$ zone of the ionized ISM and derive abundance ratios
of O/H, N/H, and N/O using the direct method. The N/O and $\alpha$/Fe abundance
patterns of these galaxies are consistent with rapid formation timescales and
ongoing strong starbursts, in accord with their high specific star-formation
rates. These results demonstrate the feasibility of using auroral [O II]
measurements for accurate metallicity studies at high redshift in a higher
metallicity regime previously unexplored with the direct method in distant
galaxies. These results also highlight the difficulty in obtaining the
measurements required for direct-method metallicities from the ground. We
emphasize the advantages that the JWST/NIRSpec instrument will bring to
high-redshift metallicity studies, where the combination of increased
sensitivity and uninterrupted wavelength coverage will yield more than an order
of magnitude increase in efficiency for multiplexed auroral-line surveys
relative to current ground-based facilities. Consequently, the advent of JWST
promises to be the beginning of a new era of precision chemical abundance
studies of the early universe at a level of detail rivaling that of local
galaxy studies.
