Implement /etc/hostname heuristic for host name capture

[SylvainJuge]

Description

Implement the extra heuristic to capture host name using /etc/hostname as fallback as added in #838 .

Java: already included in https://github.com/elastic/apm-agent-java/pull/3418 NodeJS: not needed see comment for details.

Agent Issues

• [ ] https://github.com/elastic/apm-agent-dotnet/issues/2229
• [ ] https://github.com/elastic/apm-agent-python/issues/1942
• [ ] https://github.com/elastic/apm-agent-go/issues/1548
• [ ] https://github.com/elastic/apm-agent-php/issues/1105
• [ ] https://github.com/elastic/apm-agent-ruby/issues/1426