[compressed spans] Adapt span metrics calculation

[simitt]

To make the compressed feature (elastic/apm#432) work we need to extend the intake API (#5485) and adapt the calculation of span metrics.

Adapt span metrics calculation

Since this feature is folding a set of spans into one single we need to adapt the calculation of span metrics so that the resulting metric values are still correct.

[axw]

I think this is a simple matter of updating model.Span.RepresentativeCount when decoding "span.composite":

apm-server/model/span.go

Lines 80 to 85 in 06bea7c

	// RepresentativeCount holds the approximate number of spans that
	// this span represents for aggregation. This will only be set when
	// the sampling rate is known.
	//
	// This may be used for scaling metrics; it is not indexed.
	RepresentativeCount float64

i.e. if we set that to 1/sample_rate * composite.count, then the existing span metrics aggregator should do the right thing, IIANM

[axw]

The PR #5896 is on the test plan, removing this as a duplicate