Non-unique process names in observer

In Erlang, a process can have a registered name, but it must be unique
and must be an atom.

In the http://erlang.org/pipermail/erlang-questions/2021-May/100972.html
thread, it was suggested that processes should have non-unique names
associated with them, making debugging easier.

Lukas pointed out that adding more information to the PID term was a
lot of work, but suggested extending proc_lib to add further metadata
that could be displayed by observer.

This morning, using observer, I found myself looking at a large
supervision tree with nothing but PIDs.

So I did some digging around to see what might be involved. If I get
time (I probably won't), I might put together a PR, but this'll be
useful for anyone else thinking about implementing it.

Observer gets the display name for its processes here:
https://github.com/erlang/otp/blob/OTP-24.0.2/lib/runtime_tools/src/appmon_info.erl#L708-L714;
it'd be relatively simple to get the process dictionary and look up
'$process_label' (better naming suggestions are welcome).

Adding that functionality to proc_lib, on the other hand... Well.

proc_lib offers *21* different ways to start a process, and only
spawn_opt/{2,3,4,5} allows specifying extra options, which is where
you'd probably want to put that label. I'm uncomfortable with adding
it to the spawn_option() type.

For OTP's built-in behaviours, you'd need to define an extra option to
be passed to gen_server:start_link, gen_statem:start_link, etc. That
would be less complicated (because they already take options), but is
still a chunk of work.

In the short term, I think the easiest option might be to make the
changes to observer, document it appropriately, and then just
encourage library authors to put('$process_label', MyLabel) wherever
it seems sensible.

Thoughts?

Cheers,
Roger.