add an introspection.Endpoint interface.

host/host.go

@@ -76,12 +76,14 @@ type Host interface {
 }
 
 // IntrospectableHost is implemented by Host implementations that are
-// introspectable, that is, that expose an introspection server.
+// introspectable, that is, that may have introspection capability.
 type IntrospectableHost interface {
-
-	// Introspector returns the introspection.Introspector instance, with which
-	// the caller can:
-	//  - register data providers.
-	//  - fetch introspection data.
+	// Introspector the introspector, or nil if one hasn't been registered. With
+	// it, the call can register data providers, and can fetch introspection
+	// data.
 	Introspector() introspection.Introspector
+
+	// IntrospectionEndpoint returns the introspection endpoint, or nil if one
+	// hasn't been registered.
+	IntrospectionEndpoint() introspection.Endpoint
 }

introspection/endpoint.go

@@ -0,0 +1,20 @@
+package introspection
+
+// Endpoint is the interface to be implemented by introspection
+// endpoints/servers.
+//
+// An introspection endpoint exposes introspection data over the wire via a
+// protocol and data format, e.g. WebSockets with Protobuf.
+type Endpoint interface {
+	// Start starts the introspection endpoint. It must only be called once, and
+	// once the server is started, subsequent calls made without first calling
+	// Close will error.
+	Start() error
+
+	// Close stops the introspection endpoint. Calls to Close on an already
+	// closed endpoint, or an unstarted endpoint, must noop.
+	Close() error
+
+	// ListenAddrs returns the listen addresses of this endpoint.
+	ListenAddrs() []string
+}