  We demonstrate a remarkable property of metallic Fermi liquids: the
transverse conductivity assumes a universal value in the quasi-static ($\omega
\rightarrow 0$) limit for wavevectors $q$ in the regime $l_{\rm mfp}^{-1} \ll q
\ll p_{\rm F}$, where $l_{\rm mfp}$ is the mean free path and $p_{\rm F}$ is
the Fermi momentum. This value is $(e^2/h) \mathcal{R}_{\rm FS}/q$ in two
dimensions (2D), where $\mathcal{R}_{\rm FS}$ measures the local radius of
curvature of the Fermi surface in momentum space. Even more surprisingly, we
find that U(1) spin liquids with a spinon Fermi surface have the same universal
transverse conductivity. This means such spin liquids behave effectively as
metals in this regime, even though they appear insulating in standard transport
experiments. Moreover, we show that transverse current fluctuations result in a
universal low-frequency magnetic noise that can be directly probed by a spin
qubit, such as a nitrogen-vacancy center in diamond, placed at a distance $z$
above of the 2D metal or spin liquid. Specifically the magnetic noise is given
by $C\omega \mathcal{P}_{\rm FS}/z$, where $\mathcal{P}_{\rm FS}$ is the
perimeter of the Fermi surface in momentum space and $C$ is a combination of
fundamental constants of nature. Therefore these observables are controlled
purely by the geometry of the Fermi surface and are independent of kinematic
details of the quasi-particles, such as their effective mass and interactions.
This behavior can be used as a new technique to measure the size of the Fermi
surface of metals and as a smoking gun probe to pinpoint the presence of the
elusive spinon Fermi surface in two-dimensional systems. We estimate that this
universal regime is within reach of current nitrogen-vacancy center
spectroscopic techniques for several spinon Fermi surface candidate materials.
