a client can panic etcd by passing invalid utf-8 in the client-api-version header

[rtmrtmrtmrtm]

newUnaryInterceptor() calls prometheus's WithLabelValues() on the
client-supplied client-api-version header value. WithLabelValues()
panics if the value is not valid utf-8.

I've attached a program that sends such a value to the etcd on localhost.

I've attached the resulting etcd output and backtrace.

csh# etcd/bin/etcd --version
running etcd on unsupported architecture "riscv64" since ETCD_UNSUPPORTED_ARCH is set
etcd Version: 3.5.1
Git SHA: d42e858
Go Version: go1.16.2
Go OS/Arch: linux/riscv64

etcd_2.c.txt
etcd_2.out.txt