fix: avoid IPv4 vs IPv6 ambiguity in default 'serverUrl' by using '127.0.0.1' rather than 'localhost'

[trentm]

Starting in node v17 the defaults for DNS resolution order was
changed (nodejs/node#39987) such that
dns.lookup() no longer sorted IPv4 addresses first. This impacts
usage of the default APM Server URL (the serverUrl config var),
'http://localhost:8200', when using node >=17 because the APM server
only binds to the IPv4 address by default
(elastic/apm-server#1405).

Fixes: #3045
Refs: elastic/apm#727

---

This is a start of the change. I've docs and the http-client, etc. to update. Also I'll follow up with an APM agents shared spac PR change.

Checklist

• update docs, tests
• update the HTTP client (it holds a default serverUrl as well). I think that default shouldn't live in both repos and given it is used before http-client creation in the agent repo, it should live here in the agent repo.
• spec update: Change default APM server_url to 'http://127.0.0.1:8200' to avoid IPv6 ambiguity apm#727

[apmmachine]

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• Start Time: 2022-12-08T18:10:29.707+0000

• Duration: 24 min 47 sec

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