Release metrics-lib 2.4.0

Let's release metrics-lib 2.4.0 including the legacy/trac#26159 (moved) change, so that we can make similar changes in applications using metrics-lib (legacy/trac#26162 (moved), legacy/trac#25848 (moved), legacy/trac#26169 (moved), legacy/trac#26163 (moved)).

I'm going to prepare a pre-release tarball today.