After updating for a patch version I realized a job name, passed to Push can't be a symbol. (Thank me, I wrote the tests, and they failed, not production :) )
There's no such requirement in docs (or I missed it). So I passed symbols as names, no strings.

I am eager to help, but can't deside what to do: validate that job argument passed to Prometheus::Client::Push.new is a string, or just convert it to string inside?

It fails here: https://github.com/prometheus/client_ruby/blob/main/lib/prometheus/client/push.rb#L92
So we can add job.to_s there, or right in the initialize method as @job = job.to_s.
Or validate it at the beginning and raise an error, as other methods for metrics do