Attribute extraction for root spans

[obostjancic]

Metric extraction spec

{
   "category": "span",
   "mri": "d:custom/span_attribute_4@none",
   "field": "span.data.foo",
   "tags": [{
        "key": "environment",
        "field": "span.sentry_tags.environment"
    }],
    "condition": null
},

does not work if the temp attribute is present on a root span:

// does not work
Sentry.startSpan({ name: "doSomething", op: "function" }, (rootSpan) => {
  rootSpan.setAttributes({
    foo: "bar",
  });
});

// works
Sentry.startSpan({ name: "doSomething", op: "function" }, (rootSpan) => {
  Sentry.startSpan({ name: "doSomething", op: "function" }, (innerSpan) => {
    innerSpan.setAttributes({
      foo: "bar",
    });
  });
});

The SDK sends this as a transaction event with data added into contexts.trace.data.*. Relay should perform lookup in this data:

"contexts": {
  "trace": {
    "span_id": "84ad0a67500f81ea",
    "trace_id": "cc3345cea6574367a09950eec28c0f02",
    "data": {
      "sentry.origin": "manual",
      "sentry.op": "function",
      "sentry.source": "custom",
      "sentry.sample_rate": 1,
      "foo": 1
    },
    "op": "function",
    "origin": "manual"
  }
}