Failure to parse machineType within GCP metadata

Hello,

We use Elastic APM through the Ruby agent. I'm opening this issue on this project because I think this may apply to all agents.
We're not running straight on a VM, but in a container within a Google Kubernetes Engine (k8s version 1.25.10-gke.2700 / node-pool version 1.24.12-gke.1000).

To add some context: we had to update to the 4.7.1 version of the ruby agent because we need a fix for Rails 7.1 [https://github.com/elastic/apm-agent-ruby/releases/tag/v4.7.1](https://github.com/elastic/apm-agent-ruby/releases/tag/v4.7.1)

This release also includes a fix for parsing GCP metadata : [https://github.com/elastic/apm-agent-ruby/pull/1415](https://github.com/elastic/apm-agent-ruby/pull/1415) -> [https://github.com/elastic/apm/issues/826](https://github.com/elastic/apm/issues/826)

The response of `http://metadata.google.internal/computeMetadata/v1/?recursive=true` looks like this (sanitized):

```json
{“instance”:{“attributes”:{“cluster-location”:“”,“cluster-name”:“”,“cluster-uid”:“”},“hostname”:“”,“id”:0,“serviceAccounts”:{“default”:{“aliases”:[“default”],“email”:“”,“scopes”:[“https://www.googleapis.com/auth/cloud-platform”,“https://www.googleapis.com/auth/userinfo.email”]},“our service account”:{“aliases”:[“default”],“email”:“”,“scopes”:[“https://www.googleapis.com/auth/cloud-platform”,“https://www.googleapis.com/auth/userinfo.email”]}},“zone”:“projects/.../zones/...”},“project”:{“numericProjectId”:0,“projectId”:“”}}
```

As you see, machineType is not even returned, so when [https://github.com/elastic/apm-agent-ruby/commit/c62fbc5be97ef638509b7a6d662cbfa7c62cbc4a#diff-9c3da70caafefb51aa3733d716da05e65198a507bf6d76aac25baa115c93195fR105](https://github.com/elastic/apm-agent-ruby/commit/c62fbc5be97ef638509b7a6d662cbfa7c62cbc4a#diff-9c3da70caafefb51aa3733d716da05e65198a507bf6d76aac25baa115c93195fR105) is called the parsing with `split` makes the whole thing crash.

Before there was no parsing of `machineType` so it was simply defined as `null`.

I think all agents should check for `machineType` presence before trying to parse it? Or move this change into a major version, because this is a breaking change IMO?

We plan to monkey patch the agent in the meantime.

Thanks!