attributes: distinguish multiple agent exporters sharing the same PID

[odeke-em]

If a user decides to make more than one exporter in the same process, we need to distinguish such scenarios by updating their identifier. This could happen as a bug but legitimately say if they made a proxy with a Go process as the frontend and then forked/exec'd process in the back. We should still be able to distinguish between these process and send unique information about each exporter.

Perhaps an atomic incrementer per-instance that will be sent in the attributes.