decode error when sending response.status_code as string

**Describe the bug**

According to the specification, status_code must be a number.

As part of the ops KPI review we've been investigating the following errors in ecs logs:

```
decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Response: v2.contextResponse.StatusCode: readUint64: unexpected character: �, error found in #10 byte of ...|us_code":"200"}},"sy|..., bigger context ...|.1.1"},"custom":{},"response":{"status_code":"200"}},"sync":false,"span_count":{"started":2},"ou|...,
```

`{"status_code":"200"}` is not valid format and the apm-server will fail to ingest those transactions.

**To Reproduce**

I don't have steps to reproduce, this is part of the ops KPI review and was observed in the ecs logs.

**Expected behavior**

`status_code` is sent as a number and apm-server accepts the request.

**Environment (please complete the following information)**

Most of the errors are coming from the following user agent: `elasticapm-node/3.15.0 elastic-apm-http-client/9.9.0 node/14.16.1`

This happens on multiple apm-server versions. Both 7.x and 8.x, including the latest 8.9.0.

**How are you starting the agent? (please tick one of the boxes)**

- [ ] Calling `agent.start()` directly (e.g. `require('elastic-apm-node').start(...)`)
- [ ] Requiring `elastic-apm-node/start` from within the source code
- [ ] Starting node with `-r elastic-apm-node/start`

**Additional context**

None