chore: release v2.2.0 (#175)

This commit will be tagged v2.2.0.
diff --git a/v2/gax.go b/v2/gax.go
index f634b43..28ea5fe 100644
--- a/v2/gax.go
+++ b/v2/gax.go
@@ -36,4 +36,4 @@
 package gax
 
 // Version specifies the gax-go version being used.
-const Version = "2.1.1"
+const Version = "2.2.0"