Move benchmark activation logic under ENABLE_TESTING

[mrbald]

This prevents accidental benchmarks activation if prometheus-cpp is used as a submodule of a larger project, that activates benchmarks library.

[mrbald]

An even nicer option would be to detect that prometheus-cpp is a submodule in the top level CMakeLists.txt similar to how some other projects do it, {fmt} for example. But would probably be a bigger change.

[gjasny]

Thanks for your PR!