Prevent thread name from leaking to other specs

What does this MR do and why?

The class being tested here sets the current thread name for the Rspec process. This can lead to failures in other specs.

How to set up and validate locally

bin/rspec spec/lib/gitlab/sidekiq_middleware/server_metrics_spec.rb spec/lib/gitlab/sidekiq_logging/structured_logger_spec.rb

Closes #491406 (closed) gitlab-org/quality/engineering-productivity/master-broken-incidents#8502 (closed)