Add average span size metric

[pavolloffay]

I would like to propose to add average span size metric (e.g. in bytes). This metric could be exposed in two places receiver and exporter (the attributes can change in processors).

The main motivation is to help to determine storage size and network overhead.

[tigrannajaryan]

@pjanotti thoughts on this?

[pavolloffay]

@pjanotti any update on this one?

[pjanotti]

Sorry, @pavolloffay missed this message.

After #1413 is done this will be a good addition. Not enabled by default but available to users that want it.

[bogdandrutu]

@pavolloffay I remember you did add something like this?

[pavolloffay]

This PR adds batch size bytes metric to batch processor #1270

[mx-psi]

@pavolloffay is this completed?