fix: Wrap potentially blocking code in exporter

[slowli]

What ❔

Wraps potentially blocking metrics collection in tokio::task::spawn_blocking.

Why ❔

• We already have some blocking I/O used in metric collectors (e.g., RocksDB metrics).
• Some new metrics (e.g., table sizes for Postgres) would use non-blocking I/O, which could be converted into blocking calls using Handle::block_on(). Without this wrapper, they run into the issue of calling it in a non-blocking context.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Documentation comments have been added / updated.
• Code has been formatted and linted using cargo fmt and cargo clippy.