plugin/metadata: some cleanups (#1906)

* plugin/metadata: some cleanups

Name to provider.go as that's what being defined right now in the file.
Use request.Request because that's done in variables.go anyway. Name the
main storage M, because there is no further meaning behind.

Remove superfluous methods

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

* Fix test

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

plugin/metadata/provider.go

@@ -0,0 +1,47 @@
+package metadata
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/request"
+)
+
+// Provider interface needs to be implemented by each plugin willing to provide
+// metadata information for other plugins.
+// Note: this method should work quickly, because it is called for every request
+// from the metadata plugin.
+type Provider interface {
+	// List of variables which are provided by current Provider. Must remain constant.
+	MetadataVarNames() []string
+	// Metadata is expected to return a value with metadata information by the key
+	// from 4th argument. Value can be later retrieved from context by any other plugin.
+	// If value is not available by some reason returned boolean value should be false.
+	Metadata(ctx context.Context, state request.Request, variable string) (interface{}, bool)
+}
+
+// M is metadata information storage.
+type M map[string]interface{}
+
+// FromContext retrieves the metadata from the context.
+func FromContext(ctx context.Context) (M, bool) {
+	if metadata := ctx.Value(metadataKey{}); metadata != nil {
+		if m, ok := metadata.(M); ok {
+			return m, true
+		}
+	}
+	return M{}, false
+}
+
+// Value returns metadata value by key.
+func (m M) Value(key string) (value interface{}, ok bool) {
+	value, ok = m[key]
+	return value, ok
+}
+
+// SetValue sets the metadata value under key.
+func (m M) SetValue(key string, val interface{}) {
+	m[key] = val
+}
+
+// metadataKey defines the type of key that is used to save metadata into the context.
+type metadataKey struct{}