add capability for keyed paths

[LarsxGitHub]

This PR extends CLIO with support for keyed gNMI paths. The idea is to leverage the Metric attributes to encode the keys in PathElem maps as <PathElemName><attrSep><PathElemMapKey>, e.g., interface.name and then copy over the respective value.

On the telemetry-generating side this looks as follows:

// Inside a function where you record a metric
counter, _ := meter.Int64Counter("interfaces.interface.afisafis.afisafi.value")

// Key-value pairs describing this specific metric event
metricAttributes := []attribute.KeyValue{
    attribute.String("interface.name", "Ethernet1/1/1"),
    attribute.String("interface.speed", "100Mbps"),
    attribute.String("afisafi.name", "IPV4_UNICAST"),
    attribute.String("yolo-I-can't-be-matched", "ever")
}

counter.Add(ctx, bytesReceived, metric.WithAttributes(metricAttributes...))

which translates to the following gNMI path:

[]*gpb.PathElem{
    {Name: "interfaces"},
    {Name: "interface", Key:  map[string]string{"name": "Ethernet1/1/1", "speed": "100Mbps"}},
    {Name: "afisafis"}, 
    {Name: "afisafi", Key:  map[string]string{"name": "IPV4_UNICAST"}}, 
    {Name: "value", Key:  map[string]string{"yolo-I-can't-be-matched": "ever"}},
}

Notably, to stay consistent with the existing code the following two caveats apply:
(1) if the update path starts with "container", that PathElem is skipped (as "container" is assumed to be in the prefix)
(2) If an attribute can not be matched, it is assigned to the last PathElem