exporter: Use fqdn instead of hostname only

[legraps]

Description
In distributed setups, the coordinator may point to places in different subnets. In order for clients to resolve resources behind the exporter, they need to know its FQDN instead of just the hostname.

Thus, set this to be the default when the exporter is invoked without the --hostname parameter.

Checklist

• Documentation for the feature
• Tests for the feature

• The arguments and description in doc/configuration.rst have been updated

• Add a section on how to use the feature to doc/usage.rst

• Add a section on how to use the feature to doc/development.rst

• PR has been tested

• Man pages have been regenerated

[jluebbe]

This would break many existing setups where users have a .ssh/config which matches their exporters' host names. If you need to access resources which are only accessible via the exporter, use the proxy/isolated mechanism. The proxy property already uses getfqdn().

[legraps]

Interesting, thank you for your quick feedback!
The patch was motivated by my conception that the coordinator is designed to be the central reliable point to accumulate connectivity information rather than every developer maintaining their own config. Is this wrong somehow?

As for the proxy, yes this does work and some of my colleagues use it that way. But I also expect drawbacks in forcing all traffic through the exporter and thought it as solution for some use cases only (e.g. remote/VPN, "difficult" networks outside administration control) but not the standard where name resolution into different subnets works.

[legraps]

As suggested by @Bastian-Krause, I just pushed a rework introducing a flag --fqdn to change the default when no --hostname is given.

[Emantor]

Please also update the man page under man/ and regenerate the files 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d08687c) 63.0% compared to head (6fb3fa2) 63.0%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1290     +/-   ##
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- Coverage    63.0%   63.0%   -0.1%     
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  Files         160     160             
  Lines       11901   11910      +9     
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+ Hits         7502    7504      +2     
- Misses       4399    4406      +7     

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