[META 839] Implement /etc/hostname heuristic for host name capture

[elastic-apm-tech]

See meta issue for the description and details:

• Meta issue: Implement /etc/hostname heuristic for host name capture apm#839

[picandocodigo]

The Ruby agent uses Socket.gethostname which is not dependent on /etc/hostname.

A small test with distroless:

# Dockerfile
FROM ruby:3.0.0-buster as ruby

ADD https://github.com/coord-e/magicpak/releases/latest/download/magicpak-x86_64-unknown-linux-musl /usr/bin/magicpak
RUN chmod +x /usr/bin/magicpak
RUN /usr/bin/magicpak -v $(which ruby) /bundle

WORKDIR /ruby/src
ADD . /ruby/src

FROM gcr.io/distroless/base-debian11
COPY --from=ruby /bundle /.
COPY --from=ruby /usr/lib/x86_64-linux-gnu/libyaml* /usr/lib/x86_64-linux-gnu/
COPY --from=ruby /usr/local/bin/ /usr/local/bin
COPY --from=ruby /usr/local/lib/ruby/ /usr/local/lib/ruby
COPY --from=ruby /ruby/src /

CMD ["/usr/local/bin/ruby", "-v"]
CMD ["/usr/local/bin/ruby", "app.rb"]

# app.rb
require 'socket'
puts "HOSTNAME: #{Socket.gethostname}"

$ docker run -t myapp
HOSTNAME: 0213f75455a2