We have gpr_log(GPR_DEBUG), but this needs to evaluate all arguments even if the log is thrown away, which makes it impractical for the critical path.

We've also made some use of #defines previously to enable tracing, but this is rapidly becoming unmanageable.

I'm a little in favor of having a GRPC_DEBUG environment variable (maybe with an equivalent public api) that initializes some flags that we can test in carefully selected parts of the code... And perhaps one big #define that makes all of those flags statically zero to optimize away even the checks should that prove necessary.