Support manual trace flushing interface

[kwonoj]

Describe the feature

Ref: vercel/next.js#35803 (comment)

Unlike my expectations, cleanup_env_hook will not be called when parent process exists forcefully, like process.exit(). Since our trace teardown relies on cleanup_env entirely, there's no way caller can manually flushes trace in that case.

We may want to expose an interface to close subscribers manually, along with existing cleanup hoop.

Babel plugin or link to the feature description

No response

Additional context

No response

[kwonoj]

relavant: thoren-d/tracing-chrome#10