feat(prom/exp/snmp): add support to pass multiple SNMP config files to prometheus.exporter.snmp

[hainenber]

PR Description

Which issue(s) this PR fixes

Closes #916

Notes to the Reviewer

I decided not to add another test case into snmp_test.go as it doesn't test whole component with argument but rather just a unit test for LoadSNMP function. Incoming integration test in #954 will address this.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[bastischubert]

Hi,
does this change support globbing, like the mentioned upstream PR (prometheus/snmp_exporter#970)? This will be needed to make the cloud integration have a better out of the box experience for users running with their own credentials (instead of using "public" community string)

[hainenber]

I think it does! Not yet testing it though (actually im waiting for the integration test for snmp got merged first)

[ptodev]

Suggested change

	Each file listed in in the `config_files` argument must conform to the `config_file` requirements.
	Each file listed in in the `config_files` argument must conform to the `config_file` requirements.
	Only one of `config_file` or `config_files` can be provided.

[ptodev]

It'd be simpler if we return an error if both config_files and config_file are specified. In this function we could create a config_files out of a config_file, and only pass config_files to snmp_exporter.Config. The static mode code doesn't need to support config_file because it's not used in Alloy.