Drop deprecated span.{start,http} fields

[axw]

Motivation/summary

Drop deprecated span.{start,http} fields:

• span.start.us is not used by the UI
• span.http.* fields have been copied to ECS field sets, http.* and url.*, since 7.14

A minor change to the UI is required to pick up the ECS fields: elastic/kibana#118485

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Documentation has been updated

How to test these changes

1. Send some RUM transactions and spans, check the spans have no span.start.us field
2. Send some HTTP client spans, check they have no span.http.* fields

Related issues

Closes #5995
Closes #6000

[apmmachine]

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Build stats

• Start Time: 2021-11-14T08:20:52.817+0000

• Duration: 40 min 48 sec

• Commit: 0e5dde7

Test stats 🧪

Test	Results
Failed	0
Passed	6036
Skipped	19
Total	6055

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[marclop]

Verified with 9bb5e4c:

$ curl -H "Content-type: application/x-ndjson" --data-binary @testdata/intake-v2/transactions_spans_rum_2.ndjson http://localhost:8200/intake/v2/rum/events

GET traces-apm.rum*/_search
{
  "_source": [false], 
  "fields": ["span.*"],
  "query": {
    "exists": {
      "field": "span.id"
    }
  }
}

{
  "took" : 7,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : ".ds-traces-apm.rum-default-2021.12.21-000001",
        "_id" : "1jPK230ByDpadNqDCz2H",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "transaction"
          ],
          "span.duration.us" : [
            643000
          ],
          "span.id" : [
            "aaaaaaaaaaaaaaaa"
          ],
          "span.type" : [
            "transaction"
          ]
        }
      }
    ]
  }
}

2. Send spans with http client info:

$ curl -H "Content-type: application/x-ndjson" --data-binary @testdata/intake-v2/spans.ndjson http://localhost:8200/intake/v2/events

GET traces-apm-*/_search
{
  "_source": [false], 
  "fields": ["span.*"],
  "query": {
    "exists": {
      "field": "span.id"
    }
  }
}

{
  "took" : 4,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 7,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "bDPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "GET /api/types"
          ],
          "span.action" : [
            "query.custom"
          ],
          "span.duration.us" : [
            141581
          ],
          "span.subtype" : [
            "postgresql"
          ],
          "span.id" : [
            "abcdef01234567"
          ],
          "span.type" : [
            "db"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "bTPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "GET /api/types"
          ],
          "span.duration.us" : [
            32592
          ],
          "span.id" : [
            "1234abcdef567895"
          ],
          "span.type" : [
            "request"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "bjPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "GET /api/types"
          ],
          "span.duration.us" : [
            3564
          ],
          "span.subtype" : [
            "http"
          ],
          "span.id" : [
            "0123456a89012345"
          ],
          "span.type" : [
            "request"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "bzPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "get /api/types"
          ],
          "span.action" : [
            "call"
          ],
          "span.duration.us" : [
            13980
          ],
          "span.subtype" : [
            "http"
          ],
          "span.id" : [
            "abcde56a89012345"
          ],
          "span.sync" : [
            false
          ],
          "span.type" : [
            "request"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "cDPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "SELECT FROM product_types"
          ],
          "span.destination.service.type" : [
            "db"
          ],
          "span.action" : [
            "query"
          ],
          "span.duration.us" : [
            3781
          ],
          "span.db.link" : [
            "other.db.com"
          ],
          "span.db.rows_affected" : [
            2
          ],
          "span.subtype" : [
            "postgresql"
          ],
          "span.id" : [
            "1234567890aaaade"
          ],
          "span.destination.service.name" : [
            "postgresql"
          ],
          "span.destination.service.resource" : [
            "postgresql"
          ],
          "span.sync" : [
            true
          ],
          "span.type" : [
            "db"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "cTPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.message.age.ms" : [
            1577958057123
          ],
          "span.name" : [
            "Rabbitmq receive"
          ],
          "span.action" : [
            "receive"
          ],
          "span.duration.us" : [
            141581
          ],
          "span.message.queue.name" : [
            "new_users"
          ],
          "span.subtype" : [
            "JMS"
          ],
          "span.id" : [
            "00xxx12312312312"
          ],
          "span.type" : [
            "messaging"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "cjPR230ByDpadNqDoECd",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "span.name" : [
            "SELECT FROM p_details"
          ],
          "span.action" : [
            "query"
          ],
          "span.duration.us" : [
            378191
          ],
          "span.composite.compression_strategy" : [
            "exact_match"
          ],
          "span.subtype" : [
            "postgresql"
          ],
          "span.id" : [
            "abcdef01234567"
          ],
          "span.composite.sum.us" : [
            359298
          ],
          "span.composite.count" : [
            10
          ],
          "span.type" : [
            "db"
          ]
        }
      }
    ]
  }
}