add extra heuristic for hostname

[SylvainJuge]

When using very minimal container images like the distroless, the hostname command line tool that our agents use to capture the host name is missing and can't be used.

When no value is captured for host name, the most obvious symptom is missing metrics in Kibana, the traces are not impacted.

While the HOST and HOSTNAME environment variables allow to provide a fallback, adding them to very large deployments can become an extra burden.

However, reading the content of the /etc/hostname is still possible and we should then use it to provide a value to the host name. In this case we don't know if a simple host name or an FQDN is provided.

In containers the actual value is usually the container ID or a random string thus it's not for "human consumption" but is still relevant for correlation.

In short, this just adds an extra fallback option to existing heuristics.

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[trentm]

LGTM.

(Note that the Node.js APM agent won't add this to its implementation, because it is already using the "equivalent" of hostname -f that doesn't depend on /bin/hostname existing. It currently uses Node.js's os.hostname(), which, IIRC, is using the gethostname syscall. (Earlier notes here: #805 (review))

[Mpdreamz]

Similar in .NET we don't call hostname directly.