Make intake API support compressed spans

[AlexanderWert]

To make the compressed feature (elastic/apm#432) work we need to extend the intake API and adapt the calculation of span metrics (#5595).

Extend data model

We need to introduce a new property composite in the span intake data model.
The semantics is the following.

• If this field is null, then the corresponding span is a normal span as before
• If the compressed property is not null, then it's a compressed span
  • in this case the duration represents the sum of all compressed spans
  • the composite.count represents the number of spans that have been compressed into this single span
  • composite.end is the end timestamp of the last span --> end - timestamp is the overall duration of the compressed spans including any delays between the spans
  • composite.exact_match is a boolean that indicates whether the compressed spans are exact matches of each other or of the same kind (numerous alternating, fast queries to the same backend)

{
   "name": "SELECT FROM product_types WHERE user_id=?",
   "timestamp": 1613575086000000,
   "duration": 3781,
   "composite": {
       	"count": 10,
        "end": 1613575093183900,
        "exact_match": true
    }
}

[AlexanderWert]

Resolved with #5661

[axw]

Tested in #5661