I am trying to gather counts of BGP prefixes sent to each peer from a Juniper router, using snmp_exporter. There is a SNMP metric for this: jnxBgpM2PrefixOutPrefixes, however this indexes on this do not have the local and remote peer addresses, instead have a index called jnxBgpM2PeerIndex. If i walk jnxBgpM2PeerIndex it gives me the local/remote addresses as indexes and the peer index as value.

How do i join these two things so that the out prefix count is the value, and has local/remote addresses as labels? I have tried using lookups but i think this only works if the thing you are looking up is an index, not a value?

My module config in generator.yml looks like this:
(the overrides are needed to work around a bug in the way the addresses are sent from the device)

juniper_bgp_prefixes:
    walk:
      - jnxBgpM2PrefixOutPrefixes
      - jnxBgpM2PeerIndex
      - jnxBgpM2PeerLocalAddr
      - jnxBgpM2PeerRemoteAddr
    overrides:
      jnxBgpM2PeerLocalAddr:
        type: InetAddressMissingSize
      jnxBgpM2PeerRemoteAddr:
        type: InetAddressMissingSize
    lookups:
      - source_indexes: [jnxBgpM2PeerIndex]
        lookup: jnxBgpM2PeerLocalAddr
      - source_indexes: [jnxBgpM2PeerIndex]
        lookup: jnxBgpM2PeerRemoteAddr
      - source_indexes: [jnxBgpM2PeerIndex]
        lookup: jnxBgpM2PeerRoutingInstance

An example output from this (addresses have been randomized):

# HELP jnxBgpM2PeerIndex This value is a unique index for the remote peer entry in the jnxBgpM2PeerTable - 1.3.6.1.4.1.2636.5.1.1.2.1.1.1.14
# TYPE jnxBgpM2PeerIndex gauge
jnxBgpM2PeerIndex{jnxBgpM2PeerLocalAddr="39.82.154.245",jnxBgpM2PeerRemoteAddr="39.82.154.244",jnxBgpM2PeerRoutingInstance="0"} 1
jnxBgpM2PeerIndex{jnxBgpM2PeerLocalAddr="2001:1234:0000:5000:0000:0000:0000:A5DF",jnxBgpM2PeerRemoteAddr="2001:1234:0000:5000:0000:0000:0000:A5DE",jnxBgpM2PeerRoutingInstance="0"} 3
jnxBgpM2PeerIndex{jnxBgpM2PeerLocalAddr="17.37.63.87",jnxBgpM2PeerRemoteAddr="17.37.63.86",jnxBgpM2PeerRoutingInstance="0"} 0
# HELP jnxBgpM2PeerLocalAddr The address of the local end of the peering session. - 1.3.6.1.4.1.2636.5.1.1.2.1.1.1.7
# TYPE jnxBgpM2PeerLocalAddr gauge
jnxBgpM2PeerLocalAddr{jnxBgpM2PeerLocalAddr="39.82.154.245",jnxBgpM2PeerRemoteAddr="39.82.154.244",jnxBgpM2PeerRoutingInstance="0"} 1
jnxBgpM2PeerLocalAddr{jnxBgpM2PeerLocalAddr="2001:1234:0000:5000:0000:0000:0000:A5DF",jnxBgpM2PeerRemoteAddr="2001:1234:0000:5000:0000:0000:0000:A5DE",jnxBgpM2PeerRoutingInstance="0"} 1
jnxBgpM2PeerLocalAddr{jnxBgpM2PeerLocalAddr="17.37.63.87",jnxBgpM2PeerRemoteAddr="17.37.63.86",jnxBgpM2PeerRoutingInstance="0"} 1
# HELP jnxBgpM2PeerRemoteAddr The address of the remote end of the peering session. - 1.3.6.1.4.1.2636.5.1.1.2.1.1.1.11
# TYPE jnxBgpM2PeerRemoteAddr gauge
jnxBgpM2PeerRemoteAddr{jnxBgpM2PeerLocalAddr="39.82.154.245",jnxBgpM2PeerRemoteAddr="39.82.154.244",jnxBgpM2PeerRoutingInstance="0"} 1
jnxBgpM2PeerRemoteAddr{jnxBgpM2PeerLocalAddr="2001:1234:0000:5000:0000:0000:0000:A5DF",jnxBgpM2PeerRemoteAddr="2001:1234:0000:5000:0000:0000:0000:A5DE",jnxBgpM2PeerRoutingInstance="0"} 1
jnxBgpM2PeerRemoteAddr{jnxBgpM2PeerLocalAddr="17.37.63.87",jnxBgpM2PeerRemoteAddr="17.37.63.86",jnxBgpM2PeerRoutingInstance="0"} 1
# HELP jnxBgpM2PrefixOutPrefixes The number of prefixes for a peer that are installed in that peers Adj-Ribs-Out. - 1.3.6.1.4.1.2636.5.1.1.2.6.2.1.10
# TYPE jnxBgpM2PrefixOutPrefixes gauge
jnxBgpM2PrefixOutPrefixes{jnxBgpM2PeerIndex="0",jnxBgpM2PeerLocalAddr="",jnxBgpM2PeerRemoteAddr="",jnxBgpM2PeerRoutingInstance="",jnxBgpM2PrefixCountersAfi="1",jnxBgpM2PrefixCountersSafi="1"} 7
jnxBgpM2PrefixOutPrefixes{jnxBgpM2PeerIndex="1",jnxBgpM2PeerLocalAddr="",jnxBgpM2PeerRemoteAddr="",jnxBgpM2PeerRoutingInstance="",jnxBgpM2PrefixCountersAfi="1",jnxBgpM2PrefixCountersSafi="1"} 7
jnxBgpM2PrefixOutPrefixes{jnxBgpM2PeerIndex="3",jnxBgpM2PeerLocalAddr="",jnxBgpM2PeerRemoteAddr="",jnxBgpM2PeerRoutingInstance="",jnxBgpM2PrefixCountersAfi="2",jnxBgpM2PrefixCountersSafi="1"} 4