[blob][backup] Remove blob_hash from log span

Summary:
When testing D6387, I was overwhelmed by log span metadata fields. The blob hash is very long and takes too much space when displayed with every tracing log message.
I replaced it to be displayed only once when received.

Test Plan: Both services start and display logs correctly

Reviewers: varun, jon, max

Reviewed By: max

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6388