plugin/metadata: finish documentation (#1951)

* plugin/metadata: finish documentation

Finish the README.md, add corner case in the IsLabel test and reword
some code comments slightly.

Generate the man-pages and add man/coredns-metadata.7 as well.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix test

Signed-off-by: Miek Gieben <miek@miek.nl>

3 files changed, 19 insertions, 11 deletions

diff --git a/plugin/metadata/README.md b/plugin/metadata/README.md
index aaf3375b1..55aa4d41b 100644
--- a/plugin/metadata/README.md
+++ b/plugin/metadata/README.md
@@ -8,15 +8,20 @@
 
 By enabling *metadata* any plugin that implements [metadata.Provider
 interface](https://godoc.org/github.com/coredns/coredns/plugin/metadata#Provider) will be called for
-each DNS query, at beginning of the process for that query, in order to add it's own Metadata to
+each DNS query, at beginning of the process for that query, in order to add it's own meta data to
 context.
 
-The metadata collected will be available for all plugins, via the Context parameter
-provided in the ServeDNS function. The package (code) documentation has examples on how to inspect
-and retrieve metadata a plugin might be interested in.
+The meta data collected will be available for all plugins, via the Context parameter provided in the
+ServeDNS function. The package (code) documentation has examples on how to inspect and retrieve
+metadata a plugin might be interested in.
 
-TODO: write about naming of the keys (labels).
-TODO: write about extracting and using
+The meta data is added by setting a label with a value in the context. These labels should be named
+`plugin/NAME`, where **NAME** is something descriptive. The only hard requirement the *metadata*
+plugin enforces is that the labels contains a slash. See the documentation for
+`metadata.SetValueFunc`. 
+
+The value stored is a string. The empty string signals "no meta data". See the documentation for
+`metadata.ValueFunc` on how to retrieve this.
 
 ## Syntax
 
@@ -28,7 +33,7 @@ metadata [ZONES... ]
 
 ## Plugins
 
-metadata.Provider interface needs to be implemented by each plugin willing to provide metadata
+`metadata.Provider` interface needs to be implemented by each plugin willing to provide metadata
 information for other plugins. It will be called by metadata and gather the information from all
 plugins in context.
 
@@ -36,7 +41,7 @@ Note: this method should work quickly, because it is called for every request.
 
 ## Examples
 
-There are currently no in tree plugins that write or use metadata.
+The *rewrite* plugin uses meta data to rewrite requests.
 
 ## Also See
 
diff --git a/plugin/metadata/metadata_test.go b/plugin/metadata/metadata_test.go
index 08d9b6734..be20f8770 100644
--- a/plugin/metadata/metadata_test.go
+++ b/plugin/metadata/metadata_test.go
@@ -68,20 +68,23 @@ func TestLabelFormat(t *testing.T) {
 		label   string
 		isValid bool
 	}{
+		// ok
 		{"plugin/LABEL", true},
 		{"p/LABEL", true},
 		{"plugin/L", true},
+		// fails
 		{"LABEL", false},
 		{"plugin.LABEL", false},
 		{"/NO-PLUGIN-NOT-ACCEPTED", false},
 		{"ONLY-PLUGIN-NOT-ACCEPTED/", false},
 		{"PLUGIN/LABEL/SUB-LABEL", false},
 		{"/", false},
+		{"//", false},
 	}
 
 	for _, test := range labels {
-		if IsLabel(test.label) != test.isValid {
-			t.Errorf("Label %v is expected to have this validaty : %v - and has the opposite", test.label, test.isValid)
+		if x := IsLabel(test.label); x != test.isValid {
+			t.Errorf("Label %v expected %v, got: %v", test.label, test.isValid, x)
 		}
 	}
 }
diff --git a/plugin/metadata/provider.go b/plugin/metadata/provider.go
index 276a44127..cdb9c878a 100644
--- a/plugin/metadata/provider.go
+++ b/plugin/metadata/provider.go
@@ -49,7 +49,7 @@ type Provider interface {
 // Func is the type of function in the metadata, when called they return the value of the label.
 type Func func() string
 
-// IsLabel check that the provided name looks like a valid label name
+// IsLabel checks that the provided name is a valid label name, i.e. two words separated by a slash.
 func IsLabel(label string) bool {
 	p := strings.Index(label, "/")
 	if p <= 0 || p >= len(label)-1 {