telemetry: Rename endpoint label to node label

Using a label named "endpoint" is not intuitive and requires explaining to
users that it's really just the Cluster::node value. Change the label to
"node", so that we don't need to do the explaining.

This probably breaks some existing users of the Prometheus metrics, but after
looking more at metrics recently, "endpoint" really is a thorn in my eye.

scripts/base/frameworks/telemetry/options.zeek

@@ -15,6 +15,13 @@ export {
 	## HTTP. The default value means Zeek won't expose the port.
 	const metrics_port = 0/unknown &redef;
 
+	## Every metric automatically receives a label with the following name
+	## and the metrics_endpoint_name as value to identify the originating
+	## cluster node.
+	## The label was previously hard-code as "endpoint", and that's why
+	## the variable is called the way it is, but "node" is the better label.
+	const metrics_endpoint_label = "node" &redef;
+
 	## ID for the metrics exporter. This is used as the 'endpoint' label
 	## value when exporting data to Prometheus. In a cluster setup, this
 	## defaults to the name of the node in the cluster configuration.