[APM] FIx RUM page info display

[vigneshshanmugam]

• APM Server version - 7.3.0

• RUM transaction json to the intake API. Check context.page info which has correct page URL.

{
  "transaction": {
    "context": {
      "user": {
        "id": "userId",
        "username": "username",
        "email": "email"
      },
      "custom": {
        "testContext": "testContext"
      },
      "tags": {
        "testTagKey": "testTagValue"
      },
      "page": {
        "referer": "",
        "url": "http://localhost:8000/test/e2e/general-usecase/"
      },
      "response": {
        "transfer_size": 914,
        "encoded_body_size": 621,
        "decoded_body_size": 621
      }
    },
    "span_count": {
      "started": 8
    },
    "sampled": true
  }
}

• Transaction document

"transaction": {
      "custom": {
        "testContext": "testContext"
      },
      "page": {
        "referer": "",
        "url": "http://localhost:8000/test/e2e/general-usecase/"
      },
      "span_count": {
        "started": 8
      }
}

• UI is expecting the value in the format

const url = transaction.context.page.url || transaction.url.full || NA

I am not sure if its a issue on UI or APM server side, But the code on UI has not changed for months.

[simitt]

Since 7.0 we don't store any information inside events nested under context any more. The change was implemented with https://github.com/elastic/apm-server/pull/1863/files. After moving a lot of fields to be ECS compliant, the term context was not meaningful any more and we decided to remove it.
This change probably got lost in the large amount of field changes. I'd say it needs to be fixed in the UI. @elastic/apm-ui do you agree?

[graphaelli]

Transferred over from an apm-server issue, we should change the UI to use transaction.context.page.url -> transaction.page.url. transaction.context should not actually be in use anywhere.

[sorenlouv]

@elastic/apm-ui do you agree?

Yes, agree this is a ui issue.