LogDog: Implement RegisterPrefix RPC.

common/proto/logdog/svcconfig/project.proto

Index: common/proto/logdog/svcconfig/project.proto
diff --git a/common/proto/logdog/svcconfig/project.proto b/common/proto/logdog/svcconfig/project.proto
index 1db8d3719cb558696ce93f12e7bc57ad5c15bb6e..36579dacda7c42e315391a89d61226dfaa28a9b7 100644
--- a/common/proto/logdog/svcconfig/project.proto
+++ b/common/proto/logdog/svcconfig/project.proto
@@ -33,6 +33,12 @@ message ProjectConfig {
   // This is upper-bounded by the global "archive_delay_max" parameter.
   google.protobuf.Duration max_stream_age = 4;
 
+  // The maximum amount of time after a prefix has been registered when log
+  // streams may also be registered under that prefix.
+  //
+  // See Config's "prefix_expiration" for more information.
+  google.protobuf.Duration prefix_expiration = 5;
+
   // The base Google Storage archival path, of the form: "gs://bucket/..."
   //
   // The bucket name must be included (e.g., "gs://foo"). The remainder of the