Include size of the spans in span metrics

[yvrhdn]

Is your feature request related to a problem? Please describe.

In the scenario that Tempo is used by multiple teams and that the costs of each team should be charged back, it is essential to know how much volume each team is generating.
Using the span metrics generated by the metrics-generator, it's possible to know the amount of spans per service, cluster, etc. But not the actual size of those spans.

Describe the solution you'd like

The span metrics processor should (optionally) also omit a metric traces_spanmetrics_bytes_total with the size of each received span. This metric has the same label as the existing span metrics.
This metric will behave very similar to traces_spanmetrics_calls_total, but instead of counting the spans it just adds up the sizes.

Describe alternatives you've considered

You can currently estimate the volume of each team using the average span size. This is probably close enough, but doesn't take into account that some teams might have heavier spans than others.

Additional context