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.