[Metricbeat] SNMP input or module

**Describe the enhancement:** It'd be super awesome to support hardware or systems that only have SNMP metrics exposed, or to collect information that other beats may not have access to.

**Describe a specific use case for the enhancement or feature:** I use Ubiquiti networking gear at home and it'd be great to point a beat at an SNMP endpoint, with a list of MIBs or a reference to a file with them(?), to collect data.

I have compiled metricbeat for some of their gera (as it's ARM based, which we don't officially support), so this seems like an alternative method to collecting host metrics.

There is at least [one community beat](https://github.com/isalgueiro/otilio) that does something like this. And I am also aware that there is an [SNMP input](https://www.elastic.co/guide/en/logstash/current/plugins-inputs-snmp.html) in Logstash.

Start by comparing Metricbeat's existing input or module model with the community otilio beat and the Logstash SNMP input linked in the issue. Define the supported SNMP endpoint, MIB configuration, and collected metrics; done requires an agreed implementation scope, but no repository files or tests are named.

Written by the indexing model from the issue text.