  GRB 221009A is the brightest gamma-ray burst ever detected since the
discovery of this kind of energetic explosions. However, an accurate
measurement of the prompt emission properties of this burst is very challenging
due to its exceptional brightness. With joint observations of
\textit{Insight}-HXMT and GECAM-C, we made an unprecedentedly accurate
measurement of the emission during the first $\sim$1800 s of GRB 221009A,
including its precursor, main emission (ME, which dominates the burst in flux),
flaring emission and early afterglow, in the hard X-ray to soft gamma-ray band
from $\sim$ 10 keV to $\sim$ 6 MeV. Based on the GECAM-C unsaturated data of
the ME, we measure a record-breaking isotropic equivalent energy ($E_{\rm
iso}$) of $\bf \sim 1.5 \times 10^{55}$ erg, which is about eight times the
total rest-mass energy of the Sun. The early afterglow data require a
significant jet break between 650 s and 1100 s, most likely at $\sim950$ s from
the afterglow starting time $T_{AG}$, which corresponds to a jet opening angle
of $\sim {0.7^\circ} \ (\eta_\gamma n)^{1/8}$, where $n$ is the ambient medium
density in units of $\rm cm^{-3}$ and $\eta_\gamma$ is the ratio between
$\gamma$-ray energy and afterglow kinetic energy. The beaming-corrected total
$\gamma$-ray energy $E_{\gamma}$ is $\sim 1.15 \times10^{51} \ (\eta_\gamma
n)^{1/4}$ erg, which is typical for long GRBs. These results suggest that this
GRB may have a special central engine, which could launch and collimate a very
narrowly beamed jet with an ordinary energy budget, leading to exceptionally
luminous gamma-ray radiation per unit solid angle. Alternatively, more GRBs
might have such a narrow and bright beam, which are missed by an unfavorable
viewing angle or have been detected without distance measurement.
