GOGC is ignored when config file is empty or when only unrelated config is set #16334
Description
When the config file is empty, and GOGC is set (to a value different than 75), Prometheus always uses the default GOGC value 75.
Same applies when only config unrelated to runtime: is set (runtime: block not defined)
tests to reproduce; see #15238 and uncomment the two empty config file with GOGC env var set and unrelated config and GOGC env var set cases.
see #16029 which fixed another case.