[blob] Don't log request payload in tracing span

Summary: Noticed that there's some spam in blob logs because the instrument includes full request payload in logs

Test Plan: Ran local Blob Service, called the endpoint. The payload is no longer printed.

Reviewers: will, varun, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13516