Redis/MySQL/... remote services don't differentiate hosts

**Describe the bug**

We have multiple redis servers in use - some services use their own redis available on localhost, some access a shared remote instance by its IP. The Service Map view in APM doesn't differentiate between these hosts and shows all of them as a single "redis" node used by all services. The same happens e.g., with MySQL, while HTTP remote services are correctly split by their hostname.

I'm not entirely sure if this is just the node agent not collecting all the necessary info, or a broader issue with the services spec, so feel free to move this issue to the right place if it's not directly the agent's fault.

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

- OS: Linux
- Node.js version: v14.17.1
- APM Server version: v7.15.1
- Agent version: 3.23.0

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

- [x] 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**

- `package.json` dependencies:

Click to expand

Using the `redis` module in some cases and `ioredis` in others, in case it's relevant.