fix(monitoring): Improve profiler user experience.

- Restructure profiler files for clarity:
 - profiler.go -> profiler_stackdriver.go (Stackdriver implementation)
 - null_profiler.go -> profiler_null.go (no-op implementation)
 - profiler_opts.go -> profiler.go (shared types and configuration)
- Add helpful runtime warning when profiler is requested but not built
 with the `continuous_profiler_stackdriver` build tag
- Export the `ProfilerEnvKey` constant to prevent duplication across packages

The profiler requires explicit opt-in at compile time via build tags. When
`GITLAB_CONTINUOUS_PROFILING` is set but the binary wasn't built with profiler
support, users now receive clear instructions on how to enable it.