PR: add upstream name to log message to distinguish among multiple upstreams with same server

[ecc256]

Would it be possible to add upstream name to log message to distinguish among multiple upstreams with same server.
Namely, change current message from

healthcheck: failed to receive status line from 10.0.0.1:80:

to something like

healthcheck: failed to receive status line from 10.0.0.1:80: for upstream backend

Might be easier to add error format string as healthcheck field se we can set to log the fields we need?

This is follow up to the issue.

Thank you