Check call_measurement_enabled only at startup

@yorickpeterse said in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15800#note_52114696:

To improve this further I would consider checking the flag at boot, then just reusing that value. This does mean having to HUP processes when changing the flag, but it also removes any additional overhead the flag code might have (other than a if some_variable ... end). Considering method instrumentation gets called often I think having as little overhead is more important than being able to dynamically enable/disable the feature (without a HUP/restart).

Yeah, I think that's the right call.

/cc: @pchojnacki