source/local: allow full control of label name

Make it possible for the hooks to fully define the label name to be used
(i.e. without the '<hook name>-' prefix) by prefixing the printed
feature names with a slash ('/'). This makes it possible to e.g.
override labels create by other sources.

For example having the following output from a hook:
/override_source-override_bool
/override_source-override_value=my value

will translate into the following feature labels:
feature.node.kubernetes.io/override_source-override_bool = true
feature.node.kubernetes.io/override_source-override_value = my value

README.md

@@ -193,30 +193,38 @@ available inside the Docker image so Volumes and VolumeMounts must be used if
 standard NFD images are used.
 
 The hook files must be executable. When executed, the hooks are supposed to
-print all discovered features in `stdout`, one feature per line. Hook can
+print all discovered features in `stdout`, one feature per line. Hooks can
-advertise both binary and non-binary labels, using either `<feature name>` or
+advertise both binary and non-binary labels, using either `<name>` or
-`<feature name>=<feature value>` output format. Unlike other feature sources,
+`<name>=<value>` output format.
-the source name (i.e. `local`) is not used as a prefix. The hook name is used
+
-as the prefix, instead. The full name of node label name will conform to the
+Unlike the other feature sources, the name of the hook, instead of the name of
-following convention:
+the feature source (that would be `local` in this case), is used as a prefix in
-`feature.node.kubernetes.io/<hook name>-<feature name>`.
+the label name, normally. However, if the `<name>` printed by the hook starts
-The value of the label is either `true` (for binary labels) or `<feature name>`
+with a slash (`/`) it is used as the label name as is, without any additional
+prefix. This makes it possible for the hooks to fully control the feature
+label names, e.g. for overriding labels created by other feature sources.
+
+The value of the label is either `true` (for binary labels) or `<value>`
 (for non-binary labels).
 `stderr` output of the hooks is propagated to NFD log so it can be used for
 debugging and logging.
 
-**An example:**
+**An example:**<br/>
 User has a shell script
 `/etc/kubernetes/node-feature-discovery/source.d/my-source` which has the
 following `stdout` output:
 ```
 MY_FEATURE_1
 MY_FEATURE_2=myvalue
+/override_source-OVERRIDE_BOOL
+/override_source-OVERRIDE_VALUE=123
 ```
 which, in turn, will translate into the following node labels:
 ```
 feature.node.kubernetes.io/my-source-MY_FEATURE_1=true
 feature.node.kubernetes.io/my-source-MY_FEATURE_2=myvalue
+feature.node.kubernetes.io/override_source-OVERRIDE_BOOL=true
+feature.node.kubernetes.io/override_source-OVERRIDE_VALUE=123
 ```
 
 **NOTE!** NFD will blindly run any executables placed/mounted in the hooks

source/local/local.go

@@ -60,7 +60,13 @@ func (s Source) Discover() (source.Features, error) {
 			continue
 		}
 		for feature, value := range hookFeatures {
-			features[hook+"-"+feature] = value
+			if feature[0] == '/' {
+				// Use feature name as the label as is if it is prefixed with a slash
+				features[feature[1:]] = value
+			} else {
+				// Normally, use hook name as label prefix
+				features[hook+"-"+feature] = value
+			}
 		}
 	}