[Feature Request] convert string/enum values for prometheus exporter

[patrickjahns]

We are currently using jtimon to monitor some juniper routers - with some caveats this is working fairly well.

For sensors that expose strings or enums, the exporter will omit them (see:

jtimon/prometheus_exporter.go

Lines 153 to 158 in 32fb131

	case *na_pb.KeyValue_StrValue:
	floatVal, err := strconv.ParseFloat(v.GetStrValue(), 64)
	if err != nil {
	continue
	}
	fieldValue = floatVal

)

In certain cases, it would be great to be able to convert these values and expose them. For example the bgp session-state sensor at /network-instances/network-instance/protocols/protocol/bgp/neighbors/neighbor/state/session-state exposes a ENUM (see screenshot below from explorer)

It would be great to expose those as metrics.

From the tip of my head, I see two viable options:

1. Provide a mapping for certain paths to ENUM values, a approach was done at Strings to snmp float #46

2. Move the value of that metric into a label and expose it with a constant value - POC here patrickjahns@039a7af

It would be great to receive quick feedback which the preferred option would be - I am happy to provide a pull request for this