Add support for STDIN and STDOUT to snmp_exporter_generator

[nf-brentsaner]

This being 2025, I would expect that CLI programs that read non-interactive input and write non-interactive output (e.g. not done at an interactive prompt) to support STDIN and STDOUT respectively.

Granted, one can do e.g.:

cat generator.yml | \
snmp_exporter_generator generate \
  -m mibs \
  -g /dev/stdin \
  -o /dev/stdout \
2>/tmp/err.out

to accomplish this, and it of course does work (for now), but:

• This is hacky at best (it's certainly inelegant)
• It is prone to breakage if snmp_exporter_generator ever starts printing to STDOUT during execution
  • (If implemented via proper switch arg parsing/pipe detection, e.g. -o -, that can trigger a specific global boolean that would suppress writing to STDOUT)
• I suspect non-seasoned/newbie Linux administrators to not even know the above is possible.

Thus, I suggest the following behavior instead:

• -g somefile.yml (or --generator-path="somefile.yml") should use the defined somefile.yml, regardless of STDIN.
• -g or --generator-path (no value provided) uses the default generator.yml, regardless of STDIN.
• -g - (or --generator-path="-") forces reading from STDIN.
• If no -g or --generator-path is specified, then:
  • If os.Stdin is a pipe (see here and here), use this as the input for the source generator file.
  • If it is not detected, use the default generator.yml.

Similarly, for for -o / --output-path:

• -o somefile.yml (or --output-path="somefile.yml") should use the defined somefile.yml, regardless of STDOUT.
• -o or --output-path (no value provided) uses the default snmp.yml, regardless of STDOUT.
• -o - (or --output-path="-") forces writing to STDOUT.
• If no -o or --output-path is specified, then:
  • If os.STDOUT is a pipe or redirect (see here -- basically the same as above should work, just operating on os.Stdout instead of os.Stdin), use STDOUT as the write destination.
  • If it is not detected, use the default snmp.yml.