Add Info Testable interface, implement for memory.

service/info/interface.go

Index: service/info/interface.go
diff --git a/service/info/interface.go b/service/info/interface.go
index a4541482fdebba59354e0b85681980dd715e259c..c138afa5854290ea0fcda2b2ff6b797045137029 100644
--- a/service/info/interface.go
+++ b/service/info/interface.go
@@ -36,4 +36,14 @@ type Interface interface {
 	AccessToken(scopes ...string) (token string, expiry time.Time, err error)
 	PublicCertificates() ([]Certificate, error)
 	SignBytes(bytes []byte) (keyName string, signature []byte, err error)
+
+	// Testable returns this Interface's Testable interface. Testing will return
+	// nil if testing is not supported in this implementation.
+	Testable() Testable
+}
+
+// Testable is an additional set of functions for testing instrumentation.
+type Testable interface {
+	SetVersionID(string) context.Context
+	SetRequestID(string) context.Context
 }