Currently users can register a mask function that is invoked on span creations to redact PII from the created OTEL spans.

This redaction happens (a) sync at span creation time and (b) on a span by span basis.

For users that rely on IO bound span masking, it would be great to move masking

(a) to a background thread to not block the main thread
(b) allow batch processing in masking to limit network calls made to a masking backend