plugin/metadata: tweak the docs a little
Add a simpler example that returns a static string the for metadata.
//
// Basic example:
//
-// Implement the Provider interface for a plugin:
+// Implement the Provider interface for a plugin p:
+//
+// func (p P) Metadata(ctx context.Context, state request.Request) context.Context {
+// metadata.SetValueFunc(ctx, "test/something", func() string { return "myvalue" })
+// return ctx
+// }
+//
+// Basic example with caching:
//
// func (p P) Metadata(ctx context.Context, state request.Request) context.Context {
// cached := ""
// return ctx
// }
//
-// Check the metadata from another plugin:
+// If you need access to this metadata from another plugin:
//
// // ...
// valueFunc := metadata.ValueFunc(ctx, "test/something")
