What are known metrics for TRNG? I was wondering what the known metrics are for studying the randomness of TRNG (besides NIST tests).  For example, for PUFs, there are known metrics such as uniformity, uniqueness, BER, etc.
The one metric that generically matters in cryptography for a physical entropy source is the min-entropy: the exponent of the most probable outcome, in bits.  This depends on the physics of the entropy source.  As long as it exceeds 256, you can feed a sample through a typical preimage-resistant hash function such as SHAKE256, a conditioner, and you will have what is effectively a uniform random string fit for use as cryptographic key material.

(Sometimes the physical device is called a TRNG; sometimes the composition of the physical device and the conditioner like SHAKE256 is called a TRNG.)

If your device can't produce a sample with that much min-entropy at once, but it can produce a sequence of IID samples, then you can concatenate them.  The result may be much longer than 256 bits—even if it is very far from uniform in whatever is your favorite measure of statistical distance, what matters for cryptography is only that its min-entropy be at least 256 bits.

The NIST tests hypothesize various families of probabilistic models for the entropy source, fit parameters based on a sample, and then print the entropy of the models with the fitted parameters.  These models are very simple-minded and were designed without knowledge of your device, so they are at best a way to spot-check particularly obvious predictable distributions—so obvious an engineer thought of them without even knowing what your device is.  (More details on how ‘entropy tests’ work.)

Generic measures computed on samples from your device, designed without reference to any model of the physics of your device, have very little value in studying the security of the system.  The min-entropy you advertise must be computed from a specific probabilistic model of the physics of the system to give any meaningful confidence in it.