Feature: Fingerprint-based module discovery

[SuperQ]

In order to make it easier to implement many SNMP monitoring systems do varying amounts of automatic discovery.

To solve this in the Prometheus ecosystem we could implement some amount of target feature discovery in the snmp_exporter.

Fingerprint-based module discovery

Now that we support multiple modules in a single scrape we should be able to add a fingerprint-based matching system to the generator and configuration.

Example:

modules:
  # SNMPv2-MIB for things like sysDescr, sysUpTime, etc.
  system:
    walk:
      - "SNMPv2-MIB::system"

fingerprinters:
  system:
    discovery_modules:
      - system
    matchers:
      - oid: "SNMPv2-MIB::sysObjectID"
        label: sysObjectID
        regex: "1.3.6.1.4.1.41112" # The ubnt enterprises in UBNT-MIB
        modules:
          - if_mib
          - ubiquiti_unifi

Then the target params would look like this:

http://localhost:9116/snmp?fingerprint=system&target=my-target

Since the target device returns this:

# HELP sysObjectID The vendor's authoritative identification of the network management subsystem contained in the entity - 1.3.6.1.2.1.1.2
# TYPE sysObjectID gauge
sysObjectID{sysObjectID="1.3.6.1.4.1.41112"} 1

The exporter would be able to match the UBNT-MIB matcher and include additional walks.