Option to always add PID as a span attribute

[leggebroten]

It would be nice to have option to always add the PID as an attribute

When correlating inter-process spans, it would be very helpful to always have PID as an attribute.
For instance, if I had a with_span on a process' init call, you could correlate all spans made by that process instance.

Seems like there would be two options:

• General config.exs where you turn it on for all spans
• A special :pid option (similar to :result) in the include: list.