feat: unconditionally send off a final report on LocalReporter drop

[jlizen]

📬 Issue #, if available:
Closes #113

✍️ Description of changes:
We want the local report to unconditionally flush any JFRs prior to shutting down the runtime. And then, for all other reporters, existing behavior of warning should be preserved.

I added a report_blocking method with a default implementation to non-op. That default is slightly better than the old one, since we no longer warn EVERY drop if no JFRs are omitted (previously, we unconditionally warned).

And then, the LocalReport does what you'd expect. As does the multi-reporter, it attempts each internal report's warn call on drop.

For non-local reporter usage, adds about 1 byte of memory overhead per profiler instance between the completed_normally bool + padding (which lets us improve the warning). And then, there is an extra 8 bytes for the new vtable entry, and an extra vtable pointer chase on the cancellation path.

On the non-local-reporter path (e.g. S3 reporter hitting the default no-op), try_final_report does:

1. self.state.stop() — stops async-profiler (this was going to happen anyway)
2. jfr_file.active_path() — constructs a /proc/self/fd/N PathBuf from a format string (~20 byte allocation)
3. jfr_path.metadata()?.len() — one stat syscall to check file size
4. Builds a ReportMetadata struct on the stack (two Durations + a reference + a Duration)
5. Calls report_blocking which just logs a message and returns Ok(())

We could get that further down by leaving off ReportMetadata, which we don't really use in the local reporter report_blocking anyway, but it seemed like a good idea to bake in.

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