[services] Tests - Improve logging for Blob tests

Summary:
Depends on D4584

When dealing with threads, it's good to separate logs having a prefix of a target holder in the beginning of each line.

Test Plan:

cd services
yarn run-performance-tests blob

Reviewers: tomek, varun

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4585